For simplicity, I have 2 tables in a one to many. What I'm attempting to do is get a list of all records in Table A [list of people], and the most recent document in table B [list of documents] if there is one at all.
The sql statement below is about as close as I've gotten to the goal so far, but unfortunately with the TOP 1 involved in the outer join query makes it only return 1 result for all records returned from the left side of the query.
Is there a way to get the right side of the join to execute for every record on the left side?
SELECT p.ResourceID, d.ExpiryDate FROM People p LEFT OUTER JOIN [SELECT TOP 1 ExpiryDate, ResourceID
FROM Documents
ORDER BY ExpiryDate DESC] d
Results with the query are currently: 57, 1/1/2010 69, NULL 80, NULL 120, NULL 134, NULL
When the results I'm going for should be like: 57, 1/1/2010 69, 4/26/2009 80, NULL 120, 3/8/2006 134, 9/28/1999
Any help would be greatly appreciated!
The SQL LEFT JOIN keyword returns all records from the left table [tableA], with the matching records in the right table [tableB].
The query result is NULL in the right side table when there is no match records.
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SQL LEFT JOIN Syntax
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT JOIN table_name2
ON table1.column_name1 = table2.column_name1;
or
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
Note: In some databases LEFT JOIN is called LEFT OUTER JOIN.
Sample Database Table - Books
BookId BookName BookPrice RelYear DomainName AuthorName 2 Pro Oracle SQL 168.27 2013 Security Vidyavathi 3 Sql Server Interview Questions 125 2006 Performance Nirmala 4 Getting Started With SQL 115 2009 Database Nirmala 6 The Complete Guide to SQL Server 140 2013 Administration Balan 7 Securing SQL Server 136.33 2012 Security Rishi Keshan 8 Art Of SQL 99.99 2007 Administration Siva Kumar
Sample Database Table - BookOrder
BookID OrderDate Qty DeliveryDate 1 17-09-1996 1 26-09-1996 2 22-03-2004 6 26-04-2004 4 17-07-2011 2 24-08-2011 6 23-04-1995 4 26-04-1995 8 13-08-2008 1 21-09-2008
Note:- There must be a common columns while selecting columns from multiple tables. In the "BookOrder" and "Books" both tables contains a common column called "ID".
SQL LEFT JOIN - Example
Notice that the "BookID" column in the "Books" table refers to the "BookID" in the "BookOrder" table. The relationship between the two tables above is the "BookID" column.
Then, if we run the following SQL statement [that contains an LEFT JOIN]:
SELECT
Books.BookID, Books.BookName, BookOrder.Qty, BookOrder.OrderDate
FROM Books
LEFT JOIN BookOrder
ON Books.BookID = BookOrder.BookID;
The result of above query is:
BookID BookName Qty OrderDate 2 Pro Oracle SQL 6 22-03-2004 3 Sql Server Interview Questions 4 Getting Started With SQL 2 17-07-2011 6 The Complete Guide to SQL Server 4 23-04-1995 7 Securing SQL Server 8 Art Of SQL 1 13-08-2008
Note: The SQL LEFT JOIN keyword returns all the records from the left side table [BookOrder], even if there are no matches in the right side table [Books].
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SQL Left Join
The LEFT JOIN keyword in SQL delivers all matching records [or rows], as well as records [or rows] that are available in the left table but not in the right table, as well as matched values from the right table or NULL if no row in the second table matches.
This implies that if the ON clause matches 0 [zero] entries in the right table, the join will still return a row in the output, but NULL in each of the right table's columns.
Syntax:
SELECT column_name[s]
FROM tableA
LEFT JOIN tableB ON tableA.column_name = tableB.column_name;Here, the given condition could be any given expression based on your requirement.
Example: Consider the following two tables,
Table 1 − CUSTOMERS Table is as follows.
+----+----------+-----+-----------+----------+
| ID | NAME | AGE | ADDRESS | SALARY |
+----+----------+-----+-----------+----------+
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
+----+----------+-----+-----------+----------+Table 2 − Orders Table is as follows.
+-----+---------------------+-------------+--------+
| OID | DATE | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 | 3 | 3000 |
| 100 | 2009-10-08 00:00:00 | 3 | 1500 |
| 101 | 2009-11-20 00:00:00 | 2 | 1560 |
| 103 | 2008-05-20 00:00:00 | 4 | 2060 |
+-----+---------------------+-------------+--------+Now, let us join these two tables using the LEFT JOIN as follows.
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;Output :
+----+----------+--------+---------------------+
| ID | NAME | AMOUNT | DATE |
+----+----------+--------+---------------------+
| 1 | Ramesh | NULL | NULL |
| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
| 5 | Hardik | NULL | NULL |
| 6 | Komal | NULL | NULL |
| 7 | Muffy | NULL | NULL |
+----+----------+--------+---------------------+Example 2: SQL LEFT JOIN EXAMPLE :
We'll look at two tables in this example. Employee table contains information on the employees who work in a certain department, while department table contains information about the department.
employee table
emp_no emp_name age salary dept_no
E1 Varun Singhal 27 30,000 D1
E2 Amrita Aggarwal 28 25,000 D2
E3 Ravi Anand 30 34,000 D1
E4 Nitin Saini 34 54,000 [NULL]
E5 Muskan Garg 35 65,000 [NULL]department table
dept_no dept_name location
D1 IT Delhi
D2 HR Hyderabad
D3 FINANCE RajasthanTo perform left- join on these two tables we will use the following SQL query :
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
0
Output :
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
1
Example 3: let us take two tables in this example to elaborate all the things:
CUSTOMER TABLE:
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
2
This is second table
ORDER TABLE:
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
3
join these two tables with LEFT JOIN:
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
4
Output:
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
5
Example 4: Following an OUTER JOIN with these mentioned tables, the following SQL statement may be used to extract business name and company id columns from company table and company id, item name, item unit columns from foods table:
Sample table: foods
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
6
Sample table: company
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
7
SQL Code:
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
8
Output:
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
9
Explanation:
This SQL statement would return all rows from the company table and only those rows from the foods table where the joined fields are equal and if the ON clause matches no records in the 'foods' table, the join will still return rows, but the NULL in each column of the right table.
SQL Left Join Alias
Join with same table and with table alias.
Example: I got this, hope this will be usefull to you too :
SELECT
Books.BookID, Books.BookName, BookOrder.Qty, BookOrder.OrderDate
FROM Books
LEFT JOIN BookOrder
ON Books.BookID = BookOrder.BookID;
0
Example 2: make life a little easier by using table aliases:
SELECT
Books.BookID, Books.BookName, BookOrder.Qty, BookOrder.OrderDate
FROM Books
LEFT JOIN BookOrder
ON Books.BookID = BookOrder.BookID;
1
Example 3: Selecting all fields with "SELECT *" you should explicitly name each field you need, aliasing them with AS as required. For example:
SELECT
Books.BookID, Books.BookName, BookOrder.Qty, BookOrder.OrderDate
FROM Books
LEFT JOIN BookOrder
ON Books.BookID = BookOrder.BookID;
2
And then you can reference the aliased names in your result set.
SQL Left Join Case Statement
Example 1: use CASE in a join condition. I am working in Teradata so I cannot use "DECODE".
SELECT
Books.BookID, Books.BookName, BookOrder.Qty, BookOrder.OrderDate
FROM Books
LEFT JOIN BookOrder
ON Books.BookID = BookOrder.BookID;
3
Example 2: CASE statement in LEFT JOIN:
SELECT
Books.BookID, Books.BookName, BookOrder.Qty, BookOrder.OrderDate
FROM Books
LEFT JOIN BookOrder
ON Books.BookID = BookOrder.BookID;
4
Example 3:
SELECT
Books.BookID, Books.BookName, BookOrder.Qty, BookOrder.OrderDate
FROM Books
LEFT JOIN BookOrder
ON Books.BookID = BookOrder.BookID;
5
SQL Left Join Count
The COUNT [*] function returns a number of rows in a specified table or view that includes the number of duplicates and NULL values. To return the number of rows that excludes the number of duplicates and NULL values.
Example 1: COUNT the LEFT JOIN with columns:
SELECT
Books.BookID, Books.BookName, BookOrder.Qty, BookOrder.OrderDate
FROM Books
LEFT JOIN BookOrder
ON Books.BookID = BookOrder.BookID;
6
Because the ARTICLES table doesn't have an aggregate function applied to it, you'll have to define any columns you want in the GROUP BY clause.
Example 2: left join and count:
SELECT
Books.BookID, Books.BookName, BookOrder.Qty, BookOrder.OrderDate
FROM Books
LEFT JOIN BookOrder
ON Books.BookID = BookOrder.BookID;
7
Example 2: mysql select and count left join:
SELECT
Books.BookID, Books.BookName, BookOrder.Qty, BookOrder.OrderDate
FROM Books
LEFT JOIN BookOrder
ON Books.BookID = BookOrder.BookID;
8
SQL Left Join Distinct
Example 1: SELECT DISTINCT with LEFT JOIN
selects IDs of all records of the same type, with the same dtIn date, ordered by stOut in ascending order:
SELECT
Books.BookID, Books.BookName, BookOrder.Qty, BookOrder.OrderDate
FROM Books
LEFT JOIN BookOrder
ON Books.BookID = BookOrder.BookID;
9
But it gives me an error:
ORDER BY items must appear in the select list if SELECT DISTINCT is specified
Example 2: postgresql left join distinct on:
SELECT column_name[s]
FROM tableA
LEFT JOIN tableB ON tableA.column_name = tableB.column_name;0
SQL Left Join Groupby
The GROUP BY clause can also be used with the Left Join.
Example 1: LEFT JOIN and GROUP BY :
SELECT column_name[s]
FROM tableA
LEFT JOIN tableB ON tableA.column_name = tableB.column_name;1
Example 2: The Left Join clause combined with the GROUP BY clause gives the following information: customer id, customer name, qualification, price, and date.
SELECT column_name[s]
FROM tableA
LEFT JOIN tableB ON tableA.column_name = tableB.column_name;2
Output:
SELECT column_name[s]
FROM tableA
LEFT JOIN tableB ON tableA.column_name = tableB.column_name;3
SELECT column_name[s]
FROM tableA
LEFT JOIN tableB ON tableA.column_name = tableB.column_name;4
Output:
SELECT column_name[s]
FROM tableA
LEFT JOIN tableB ON tableA.column_name = tableB.column_name;5
SELECT column_name[s]
FROM tableA
LEFT JOIN tableB ON tableA.column_name = tableB.column_name;6
Output:
SELECT column_name[s]
FROM tableA
LEFT JOIN tableB ON tableA.column_name = tableB.column_name;7
SQL Left Join Multiple Tables
If you need to LEFT JOIN more than two tables to obtain the information you need for a certain analysis.
In SQL, we usually use Join to create a new table. The order in which we provide tables is more important, the tables from which we need to collect all records, tuples, or rows from the left table, and only those records corresponding from the right table.
The LEFT JOIN procedure in SQL cascades across all joins in a query, which means If you utilize an LEFT JOIN, the following tables should often be left-joined as well.
The maximum number of tables that can be joined in a single SQL statement is determined by the RDMS. A connect with more than four or five tables, on the other hand, is not recommended. The number of tables supplied in the FROM clause determines the order of magnitude and complexity of the join.
Syntax:
SQL Left Join multiple tables Syntax
Here, Table1 would be consider as a left table and Table2 would be consider as a right table. similarly for second left join Table2 is considered as left table for table 3 , Table1.column1, Table1.column2, Table2.column1, are the name of the columns which you want to retrieve separated by comma.
SELECT column_name[s]
FROM tableA
LEFT JOIN tableB ON tableA.column_name = tableB.column_name;8
Example 1: SQL Left Join with three tables example:
To get the combine records of patients, doctors and their laboratory use SQL Left join multiple tables.
SELECT column_name[s]
FROM tableA
LEFT JOIN tableB ON tableA.column_name = tableB.column_name;9
The above Left Outer join on three tables query provides a set of all records from the patient table, including patient id, patient name, doctor name, patient age, patient gender, patient address, patient diseases, and matching records from the doctor and laboratory tables, including doctor address, doctor city with lab no, lab report date, and lab report amount.
Example 2: You can refer same 2 tables with following additional table for fetching data in 3 tables.
Employee Table:
+----+----------+-----+-----------+----------+
| ID | NAME | AGE | ADDRESS | SALARY |
+----+----------+-----+-----------+----------+
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
+----+----------+-----+-----------+----------+0
Department Table:
+----+----------+-----+-----------+----------+
| ID | NAME | AGE | ADDRESS | SALARY |
+----+----------+-----+-----------+----------+
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
+----+----------+-----+-----------+----------+1
Salary Table :
+----+----------+-----+-----------+----------+
| ID | NAME | AGE | ADDRESS | SALARY |
+----+----------+-----+-----------+----------+
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
+----+----------+-----+-----------+----------+2
Problem Statement:
We need to fetch data from Employee table with its salary and associated department. If Department is not associated with Employee need to show that Blank.
+----+----------+-----+-----------+----------+
| ID | NAME | AGE | ADDRESS | SALARY |
+----+----------+-----+-----------+----------+
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
+----+----------+-----+-----------+----------+3
Output :
+----+----------+-----+-----------+----------+
| ID | NAME | AGE | ADDRESS | SALARY |
+----+----------+-----+-----------+----------+
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
+----+----------+-----+-----------+----------+4
Query Explanation Step-by-Step :
- Step 1 : To retrieve Employee and its associated Salary, the Employee and Salary tables are joined together.
- Step 2 : To obtain the department linked with an employee, use that set and join it to the Department table.
Example 3:
Consider two tables:
1. Employee [Left Table] :
+----+----------+-----+-----------+----------+
| ID | NAME | AGE | ADDRESS | SALARY |
+----+----------+-----+-----------+----------+
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
+----+----------+-----+-----------+----------+5
2. Projects [Right Table] :
+----+----------+-----+-----------+----------+
| ID | NAME | AGE | ADDRESS | SALARY |
+----+----------+-----+-----------+----------+
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
+----+----------+-----+-----------+----------+6
To Join these two tables and to obtain common information we need to use the following query
+----+----------+-----+-----------+----------+
| ID | NAME | AGE | ADDRESS | SALARY |
+----+----------+-----+-----------+----------+
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
+----+----------+-----+-----------+----------+7
Once after obtaining the table as you can see that the Emp_id who is not assigned for a project who’s Project_Assigned_date has became NULL and No_of_hours_worked also became NULL cause the Employee has not assigned anything to do.
Left Join means that, based on the above tables, it collected data from both table rows that match and returned NULL values for the rows whose data is not present in Table 2 since we need to consider all of the data in the Left table.
Example 4: Consider the following scenario. We'd like to look at how our recent promotional campaign has influenced our customers behaviour.
+----+----------+-----+-----------+----------+
| ID | NAME | AGE | ADDRESS | SALARY |
+----+----------+-----+-----------+----------+
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
+----+----------+-----+-----------+----------+8
To do this, we need to combine the data about customers, sales, and promotions.
+----+----------+-----+-----------+----------+
| ID | NAME | AGE | ADDRESS | SALARY |
+----+----------+-----+-----------+----------+
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
+----+----------+-----+-----------+----------+9
Output:
+-----+---------------------+-------------+--------+
| OID | DATE | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 | 3 | 3000 |
| 100 | 2009-10-08 00:00:00 | 3 | 1500 |
| 101 | 2009-11-20 00:00:00 | 2 | 1560 |
| 103 | 2008-05-20 00:00:00 | 4 | 2060 |
+-----+---------------------+-------------+--------+0
As you can see, we kept track of all of our clients via an LEFT JOIN, independent of purchase history or participation in promotional programmes. Customer 1 is, for example, a result of the join despite having made no transactions or received the promotional mail. Customer 4 has purchased a book but has not gotten any promotional messages, and Customer 5 has received a promotional message but has not purchased anything. Finally, the outcome includes customers who have made purchases and received promotional communications [Customers 2 and 3].
SQL Left Join only First Match
Example 1: Writing a query that joins two tables and selects only first matches from the second table for every result from the first table is an old task for SQL developers. In my scenario, I need to merge the client's initial phone number from the Phones field with his or her name from the Clients table.
After doing some research, I came to three different conclusions.
1. left Join with SELECT TOP 1 subquery
+-----+---------------------+-------------+--------+
| OID | DATE | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 | 3 | 3000 |
| 100 | 2009-10-08 00:00:00 | 3 | 1500 |
| 101 | 2009-11-20 00:00:00 | 2 | 1560 |
| 103 | 2008-05-20 00:00:00 | 4 | 2060 |
+-----+---------------------+-------------+--------+1
Example 2: I'm running a query against a lot of enormous tables [rows and columns] with a lot of joins, but one of the tables contains some duplicate rows of data, which is causing problems for my query. I can't fix the data because it's a read-only realtime feed from another department, but I'm attempting to avoid problems in my query because of it. used in a left join, see below:
+-----+---------------------+-------------+--------+
| OID | DATE | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 | 3 | 3000 |
| 100 | 2009-10-08 00:00:00 | 3 | 1500 |
| 101 | 2009-11-20 00:00:00 | 2 | 1560 |
| 103 | 2008-05-20 00:00:00 | 4 | 2060 |
+-----+---------------------+-------------+--------+2
SQL Left Join only One Column
The sql left join joins tables, but we just utilise one column here.
Syntax:
mysql join only one column
+-----+---------------------+-------------+--------+
| OID | DATE | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 | 3 | 3000 |
| 100 | 2009-10-08 00:00:00 | 3 | 1500 |
| 101 | 2009-11-20 00:00:00 | 2 | 1560 |
| 103 | 2008-05-20 00:00:00 | 4 | 2060 |
+-----+---------------------+-------------+--------+3
Example 1: Following an OUTER JOINING with these mentioned tables, the following SQL statement can be used to extract business name and company id columns from company table and company id, item name, item unit columns from foods table:
Sample table: foods
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
6
Sample table: company
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
7
Output:
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
9
Explanation:
This SQL statement would return all rows from the company table and only those rows from the foods table where the joined fields are equal and if the ON clause matches no records in the 'foods' table, the join will still return rows, but the NULL in each column of the right table.
SQL Left Join Same Table
Example 1: Mysql LEFT JOINing same table twice
+-----+---------------------+-------------+--------+
| OID | DATE | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 | 3 | 3000 |
| 100 | 2009-10-08 00:00:00 | 3 | 1500 |
| 101 | 2009-11-20 00:00:00 | 2 | 1560 |
| 103 | 2008-05-20 00:00:00 | 4 | 2060 |
+-----+---------------------+-------------+--------+7
+-----+---------------------+-------------+--------+
| OID | DATE | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 | 3 | 3000 |
| 100 | 2009-10-08 00:00:00 | 3 | 1500 |
| 101 | 2009-11-20 00:00:00 | 2 | 1560 |
| 103 | 2008-05-20 00:00:00 | 4 | 2060 |
+-----+---------------------+-------------+--------+8
+-----+---------------------+-------------+--------+
| OID | DATE | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 | 3 | 3000 |
| 100 | 2009-10-08 00:00:00 | 3 | 1500 |
| 101 | 2009-11-20 00:00:00 | 2 | 1560 |
| 103 | 2008-05-20 00:00:00 | 4 | 2060 |
+-----+---------------------+-------------+--------+9
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;0
So I have two tables and want to see a column with the customer's name, then a column with the total number of dogs purchased, and then a column for cats, and this is what I ended up with.
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;1
Output:
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;2
Example 2: The staffs table twice: one as e for the employees and the other as m for the managers. The join predicate matches employee and manager relationship using the values in the e.manager_id and m.staff_id columns.
Because of the INNER JOIN effect, Fabiola Jackson does not appear in the employee column. If you substitute the INNER JOIN clause in the following query with the LEFT JOIN clause, you'll obtain the following result set, which includes Fabiola Jackson in the employee column:
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;3
SQL Left Join Subquery
Because no indexes may be used on a temporary table in memory, using subqueries in JOIN operations should typically be avoided if you can rewrite the query in a different way. utilising a left join on a subquery to get a list of towns
MySQL's ability to correlate subqueries is severely limited. A correlated value, for example, cannot be nested more than one level deep, nor can it be utilised in a join's ON clause. As a result, we'll need to add an extra join to our subqueries.
It's worth noting that the join keyword's left and right tables must both yield a common key that may be used to join the tables.
Example 1: In Access, I can create a query [tblCurrentNamesInTown]:
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;4
then a new query [tblTownData Without Matching Current]:
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;5
Example 2: We'll need to join game to itself in the subqueries and utilise the values of a joined table rather than the correlated values to rewrite the aggregate query as subqueries:
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;6
Example 3: We've taken the tables Movies and movie_genre, which we built previously, as samples. Let's have a look at the query:
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;7
SQL Left Join Sum
Example 1: You're combining table 'goods' with two other tables, each of which has a one-to-many relationship with the 'goods' table. When they're combined, a new set of rows appears, so if there are two photos, shop products will appear twice.
The simplest approach to handle this is to determine the statistics of the sub-tables before joining them and counting unique items using distinct counting, therefore your query should be:
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;8
Example 2: Calculating the fuel consumption grouped by the column CarType. To get the fuel consumption I want to calculate the distance and fuel quantity for each car, then sum that up grouped by the column CarType.
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;9
Example 3: Left join with sum clause:
+----+----------+--------+---------------------+
| ID | NAME | AMOUNT | DATE |
+----+----------+--------+---------------------+
| 1 | Ramesh | NULL | NULL |
| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
| 5 | Hardik | NULL | NULL |
| 6 | Komal | NULL | NULL |
| 7 | Muffy | NULL | NULL |
+----+----------+--------+---------------------+0
agents_data table is included in the query data source. None of its field is used in a query [except join condition] and it cannot affect to the query result because of left join [except unwanted multiplying].
Example 4:
+----+----------+--------+---------------------+
| ID | NAME | AMOUNT | DATE |
+----+----------+--------+---------------------+
| 1 | Ramesh | NULL | NULL |
| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
| 5 | Hardik | NULL | NULL |
| 6 | Komal | NULL | NULL |
| 7 | Muffy | NULL | NULL |
+----+----------+--------+---------------------+1
or
+----+----------+--------+---------------------+
| ID | NAME | AMOUNT | DATE |
+----+----------+--------+---------------------+
| 1 | Ramesh | NULL | NULL |
| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
| 5 | Hardik | NULL | NULL |
| 6 | Komal | NULL | NULL |
| 7 | Muffy | NULL | NULL |
+----+----------+--------+---------------------+2
The id Column is now removed [probably not what you want to group on], I included the awardtypeid in the choose in the first case, but you must also include it in the group by.
SQL Left Join Union and Unionall
Example 1: SQL UNION ALL LEFT JOIN:
+----+----------+--------+---------------------+
| ID | NAME | AMOUNT | DATE |
+----+----------+--------+---------------------+
| 1 | Ramesh | NULL | NULL |
| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
| 5 | Hardik | NULL | NULL |
| 6 | Komal | NULL | NULL |
| 7 | Muffy | NULL | NULL |
+----+----------+--------+---------------------+3
Example 2: Subquery that you've named 'bl_bands' doesn't have bl_bands.id because all the union joins don't include the bl_bands table. Try adding joins to each union, if my assumption on all your data is correct:
+----+----------+--------+---------------------+
| ID | NAME | AMOUNT | DATE |
+----+----------+--------+---------------------+
| 1 | Ramesh | NULL | NULL |
| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
| 5 | Hardik | NULL | NULL |
| 6 | Komal | NULL | NULL |
| 7 | Muffy | NULL | NULL |
+----+----------+--------+---------------------+4
It appears that you copied/pasted several SELECT statements/column names but did not include the joining required to obtain the results.
SQL Left Join vs Inner Join
When you need all records from the "left" table, regardless of whether they have a pair in the "right" table, use INNER JOIN, and when you need all records from the "right" table, use LEFT JOIN. You'll need to use CROSS JOIN if you need all records from both tables, regardless of whether they have pair [or simulate it using LEFT JOINs and UNION].
INNER JOIN
Inner join only returns matched rows from both tables involved in the join; non-matching rows are removed.
The INNER JOIN command selects only rows that satisfy an ON condition. There will be no results displayed if there are no rows that match the ON condition.
Example 1: Inner joins two tables together to find common records. We have "ClassID" in the tclass database and "ClassID" in the tstudent table in the instance above. The following query is used to get a list of students in Class "10th."
+----+----------+--------+---------------------+
| ID | NAME | AMOUNT | DATE |
+----+----------+--------+---------------------+
| 1 | Ramesh | NULL | NULL |
| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
| 5 | Hardik | NULL | NULL |
| 6 | Komal | NULL | NULL |
| 7 | Muffy | NULL | NULL |
+----+----------+--------+---------------------+5
As a result, the aforementioned query is used to retrieve a list of students in Class – "10th." So all students are saved in the "tstudent" table, regardless of their classes, and all classes are saved in the "tclass" table. As a result of the query, matched records from the tstudent and tclass tables are found and shown. The term "predicate" was introduced in the introduction chapter; it is "ON c.ClassID = s.ClassID" in the above query, which is an important element of the join.
Note : There are multiple keys associated to each table like in tStudent table have “StudentID” as a primary key field and “ClassID” as a foreign key which inturn refer to the table – “tClass”.
The above query can be rewritten without using inner join like below but the performance will be impacted compared to inner join :
+----+----------+--------+---------------------+
| ID | NAME | AMOUNT | DATE |
+----+----------+--------+---------------------+
| 1 | Ramesh | NULL | NULL |
| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
| 5 | Hardik | NULL | NULL |
| 6 | Komal | NULL | NULL |
| 7 | Muffy | NULL | NULL |
+----+----------+--------+---------------------+6
LEFT JOIN
Only the matching rows from both tables, containing non-matching rows, as well as non-matching rows from the left table, are returned in the Left Join or Left Outer Join.
LEFT JOIN returns all data from the first table, regardless of whether there are any matches with the second table, and NULL fills in the gaps.
Performance difference between INNER JOIN and LEFT JOIN
INNER JOIN and LEFT JOIN performance is affected by the number of JOINS performed and whether or not the columns are indexed. Performing entire 9 to 10 table scans for each JOIN could also slow down the operation. When some of the tables are relatively small, say less than 10 rows, or when the tables lack enough indexes to cover the query, LEFT JOIN may be faster than INNER JOIN. As a result, the circumstances are crucial.
Example: Below query is used to fetch the all the classes and the students are in that class.
+----+----------+--------+---------------------+
| ID | NAME | AMOUNT | DATE |
+----+----------+--------+---------------------+
| 1 | Ramesh | NULL | NULL |
| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
| 5 | Hardik | NULL | NULL |
| 6 | Komal | NULL | NULL |
| 7 | Muffy | NULL | NULL |
+----+----------+--------+---------------------+7
Above can be rewritten using “LEFT OUTER JOIN” as :
+----+----------+--------+---------------------+
| ID | NAME | AMOUNT | DATE |
+----+----------+--------+---------------------+
| 1 | Ramesh | NULL | NULL |
| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
| 5 | Hardik | NULL | NULL |
| 6 | Komal | NULL | NULL |
| 7 | Muffy | NULL | NULL |
+----+----------+--------+---------------------+8
As illustrated in the accompanying diagram, all rows from "tclass" are retrieved, as are the students of the various classes. If no students are identified in that class, the class will still be retrieved from the left table, with NULL displayed in place of "StudentID" and "StudentName."
SQL Left Join vs Left Outer Join
There is no distinction between LEFT JOIN and LEFT OUTER JOIN. Despite the fact that both yield the same output and performance, I prefer to use LEFT OUTER JOIN over just LEFT JOIN. As it is more readable and leaves no misunderstanding. Please let us know which one you prefer and why.
The PostgreSQL LEFT JOIN joins two tables and retrieves results based on a criteria that is met in both tables, with unmatched rows also obtainable from the table created before the JOIN clause.
A left outer join will contain the entire set of records from the first table, together with the matching results in the related table. If no matching results are found, the right side will contain a null. The usage of a 'where' clause is used to produce only the records in the left table and not the right table.
Syntax:
+----+----------+--------+---------------------+
| ID | NAME | AMOUNT | DATE |
+----+----------+--------+---------------------+
| 1 | Ramesh | NULL | NULL |
| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
| 5 | Hardik | NULL | NULL |
| 6 | Komal | NULL | NULL |
| 7 | Muffy | NULL | NULL |
+----+----------+--------+---------------------+9
If a record in table1 fulfils the WHERE clause but not the ON condition, an extra table2 entry is created with all columns set to NULL.
Example 1: Let us prove with examples that there is no difference between LEFT JOIN and LEFT OUTER JOIN.
LEFT JOIN
emp_no emp_name age salary dept_no
E1 Varun Singhal 27 30,000 D1
E2 Amrita Aggarwal 28 25,000 D2
E3 Ravi Anand 30 34,000 D1
E4 Nitin Saini 34 54,000 [NULL]
E5 Muskan Garg 35 65,000 [NULL]0
LEFT OUTER JOIN
emp_no emp_name age salary dept_no
E1 Varun Singhal 27 30,000 D1
E2 Amrita Aggarwal 28 25,000 D2
E3 Ravi Anand 30 34,000 D1
E4 Nitin Saini 34 54,000 [NULL]
E5 Muskan Garg 35 65,000 [NULL]1
Example 2: The LEFT join in PostgreSQL retrieves the entire set of records from the left, and the matching records [depending on availability] from the right. When no matching occurs, the result is NULL on the right side.
emp_no emp_name age salary dept_no
E1 Varun Singhal 27 30,000 D1
E2 Amrita Aggarwal 28 25,000 D2
E3 Ravi Anand 30 34,000 D1
E4 Nitin Saini 34 54,000 [NULL]
E5 Muskan Garg 35 65,000 [NULL]2
OR
emp_no emp_name age salary dept_no
E1 Varun Singhal 27 30,000 D1
E2 Amrita Aggarwal 28 25,000 D2
E3 Ravi Anand 30 34,000 D1
E4 Nitin Saini 34 54,000 [NULL]
E5 Muskan Garg 35 65,000 [NULL]3
In the previous example, the item no I8 of the item_table does not exist in the invoice table, so a new row in the invoice table was constructed for these rows of the item table and set to NULL.
SQL Left Join vs Not Exists
LEFT JOIN / IS NULL performs worse in both cases because it either performs an extra table lookup or does not return on the first match.
NOT EXISTS is a simple function that just checks equality and returns TRUE or FALSE on the first hit or miss.
The most significant difference is [as written], the SELECT *, not the join vs. not exists.
Syntax:
emp_no emp_name age salary dept_no
E1 Varun Singhal 27 30,000 D1
E2 Amrita Aggarwal 28 25,000 D2
E3 Ravi Anand 30 34,000 D1
E4 Nitin Saini 34 54,000 [NULL]
E5 Muskan Garg 35 65,000 [NULL]4
LEFT JOIN / IS NULL will return TRUE only if no row in table2 matches the equality condition, so it will behave similarly to NOT EXISTS.
Example 1: Is NOT EXISTS or LEFT OUTER JOIN...IS NULL more efficient for finding records in one table that aren't in another? Specifically, I'm trying to figure out which of the two queries below is better:
emp_no emp_name age salary dept_no
E1 Varun Singhal 27 30,000 D1
E2 Amrita Aggarwal 28 25,000 D2
E3 Ravi Anand 30 34,000 D1
E4 Nitin Saini 34 54,000 [NULL]
E5 Muskan Garg 35 65,000 [NULL]5
or
emp_no emp_name age salary dept_no
E1 Varun Singhal 27 30,000 D1
E2 Amrita Aggarwal 28 25,000 D2
E3 Ravi Anand 30 34,000 D1
E4 Nitin Saini 34 54,000 [NULL]
E5 Muskan Garg 35 65,000 [NULL]6
Example 2: LEFT JOIN / IS NULL
emp_no emp_name age salary dept_no
E1 Varun Singhal 27 30,000 D1
E2 Amrita Aggarwal 28 25,000 D2
E3 Ravi Anand 30 34,000 D1
E4 Nitin Saini 34 54,000 [NULL]
E5 Muskan Garg 35 65,000 [NULL]7
Example 3: On the first example, you get all columns from both A and B, whereas in the second example, you get only columns from A.
To achhieve this Perform the two test SELECT statement variants:
emp_no emp_name age salary dept_no
E1 Varun Singhal 27 30,000 D1
E2 Amrita Aggarwal 28 25,000 D2
E3 Ravi Anand 30 34,000 D1
E4 Nitin Saini 34 54,000 [NULL]
E5 Muskan Garg 35 65,000 [NULL]8
SQL Left Join vs Notin
The LEFT JOIN command returns all records from the first left table, matched records from the second right table, and NULL values from the right side for records from the left table that do not have a match in the right table.
The WHERE clause of the SQL NOT IN command can have several values. It can compare particular column values from the first table to other column values in the second table or a subquery and return all values from the first table that aren't found in the second table, without filtering for distinct values. The NOT IN command considers NULL as a value and returns it.
Example 1: Using NOT IN or LEFT JOIN WHERE IS NULL in a SQL query achieves the same result. For instance:
emp_no emp_name age salary dept_no
E1 Varun Singhal 27 30,000 D1
E2 Amrita Aggarwal 28 25,000 D2
E3 Ravi Anand 30 34,000 D1
E4 Nitin Saini 34 54,000 [NULL]
E5 Muskan Garg 35 65,000 [NULL]9
Example 2: Just for fun, choose one of the options LEFT JOIN or NOT IN. Review the following two queries for Join Better Performance if you need to refer to the query that displays the given clauses.
dept_no dept_name location
D1 IT Delhi
D2 HR Hyderabad
D3 FINANCE Rajasthan0
The first query with a NOT IN consumes 20% of the execution plan's resources, while the LEFT JOIN consumes 80% of the execution plan's resources. In this case, the NOT IN clause is preferable to the LEFT JOIN clause. Please keep in mind that this is a specific case and not a general conclusion. Many elements can influence your outcome. Please let me know if you guessed correctly or incorrectly.
Example 3: Performance of NOT IN and LEFT JOIN:
dept_no dept_name location
D1 IT Delhi
D2 HR Hyderabad
D3 FINANCE Rajasthan1
SQL Left Join vs Right Join
LEFT JOIN and RIGHT JOIN are the two main types of joins in MySQL. The inclusion of non-matched rows is the key distinction between both joins.
Both are outer joins, which produce a table including the matched data from the two tables, as well as the remaining rows of the left table and matching rows from the right. If there is no matching row on the right side table, the result-set will be null.
The right join, on the other hand, is an outer join that produces a table including the matched data from the two tables being joined, as well as the remaining rows of the right table and matching rows from the left table. If no matching row on the left side table exists, the result-set will have a null value.
When we say left join, we're talking about the left outer join, and when we say right join, we're talking about the right outer join.
Example 1: Suppose you want the following result:
dept_no dept_name location
D1 IT Delhi
D2 HR Hyderabad
D3 FINANCE Rajasthan2
You could do it this way:
dept_no dept_name location
D1 IT Delhi
D2 HR Hyderabad
D3 FINANCE Rajasthan3
Or this way:
dept_no dept_name location
D1 IT Delhi
D2 HR Hyderabad
D3 FINANCE Rajasthan4
To put it another way, when you use LEFT OUTER JOIN, you obtain all rows from the table on the left. When you use RIGHT OUTER JOIN, you obtain all rows from the table on the right.
Example 2: When the tables are student and location, the SQL statement for left join is:
dept_no dept_name location
D1 IT Delhi
D2 HR Hyderabad
D3 FINANCE Rajasthan5
whereas, the SQL statement for right join is:
dept_no dept_name location
D1 IT Delhi
D2 HR Hyderabad
D3 FINANCE Rajasthan6
Example 3: LEFT JOIN Example:
Following SQL statement returns the matching records from both tables using the LEFT JOIN query:
dept_no dept_name location
D1 IT Delhi
D2 HR Hyderabad
D3 FINANCE Rajasthan7
RIGHT JOIN Example
Following SQL statement returns the matching records from both tables using the RIGHT JOIN query:
dept_no dept_name location
D1 IT Delhi
D2 HR Hyderabad
D3 FINANCE Rajasthan8
Example 4: There are certain records in both tables that have no matching records. For example, the developer's table has a record with the name Siddharth and the team id 7, but the team's table has no record with that id. Similarly, the team's table contains records for departments CRM, Bug Solver, and Document with ids 4,5 and 6 that do not match any records in table developers.
Now, let us perform the left join on tables teams and developers in the following way:
dept_no dept_name location
D1 IT Delhi
D2 HR Hyderabad
D3 FINANCE Rajasthan9
Now, let us perform the right join on tables teams and developers in the following way:
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
00
SQL Left Join in Where
Before using LEFT JOIN, it uses the same filtering mechanism. WHERE CLAUSE can also be combined with LEFT JOIN and ON Clause for this reason.
WHERE CLAUSE filters the rows in a table or several tables, as you may know.
In an LEFT JOIN, the WHERE CLAUSE filters the records of either the LEFT Table or the RIGHT Table first, before performing the join with the ON Clause.
Syntax :
Here is the syntax of how you can use WHERE CLAUSE with SQL LEFT JOIN.
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
01
Example 1: Let's say we only want to join the Clients and Orders tables for customers whose accounts aren't suspended. The SELECT statement with LEFT JOIN and WHERE CLAUSE is as follows:
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
02
Example 2: use of where in left join in mysql:
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
03
Example 3: add a When clause to the query to extract only the data from "Table2" where the ID is less than 4, for example:
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
04
SQL Left Join with Top 1
When we use a left join, the results will favour the left table, but if we only want the first few records, we may use a top 1 left join.
Example 1: There may be numerous rows with the same marker key in dps markers, but we only want to join against the first. If I remove the top 1 and ORDER BY, I receive a result for mbg.marker value, however if I execute the query as is, it always returns null.
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
05
Example 2: Select TOP 1 records from LEFT JOIN with SQL:
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
06
Output:
SELECT column_name1, column_name2, ...column_nameN
FROM table_name1
LEFT OUTER JOIN table_name2
ON table1.column_name1 = table2.column_name1;
07
Example 3: Instead of an LEFT JOIN, the solution was to perform an OUTER APPLY. An outer apply, like a sub select, allows you to build a sub query that references the ID from the original table. Because this is essentially a join, you have access to numerous columns.
Consider a table with customers and addresses, where each customer can have many addresses.
Customer [CustomerID, FirstName, LastName]
Address [AddressID, CustomerID, Line1, Line2, Town, County, Country, DateAdded]
The goal is to obtain a list of customers as well as their most recent address. Using an OUTER APPLY, we can link the two tables and obtain the most current address as follows:
How do I SELECT the top 1 row in SQL join?
4 Ways to Join Only The First Row in SQL.
Use Correlated Subqueries when the foreign key is indexed..
Use a Complete Subquery when you don't have indexes..
Use Nested Subqueries if you have an ordered ID column..
Use Nested Subqueries if you have an ordered ID column..
Use Window Functions if you need more control..
What is the difference between SELECT top 1 and SELECT 1?
SELECT TOP 1 1 will select exactly 0 or 1 1 s. SELECT 1 will select 1 exactly N rows, where N is the number of rows that match your criteria. In your case, it is looking for the first [ TOP 1 ] worker that does not have a manager.
When would you use a left join?
A join combines the set of two tables only. A left join is used when a user wants to extract the left table's data only. Left join not only combines the left table's rows but also the rows that match alongside the right table.
Is LEFT join better than subquery?
A LEFT [OUTER] JOIN can be faster than an equivalent subquery because the server might be able to optimize it better—a fact that is not specific to MySQL Server alone. Prior to SQL-92, outer joins did not exist, so subqueries were the only way to do certain things.