T-SQL - Joining Tables
The MS SQL Server Joins clause is used to combine records from two or more tables in a database. A JOIN is a means for combining fields from two tables by using values common to each.
Consider the following two tables, (a) 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
(b) Another table is ORDERS as follows −
OID DATE CUSTOMER_ID AMOUNT 100 2009-10-08 00:00:00.000 3 1500.00 101 2009-11-20 00:00:00.000 2 1560.00 102 2009-10-08 00:00:00.000 3 3000.00 103 2008-05-20 00:00:00.000 4 2060.00
Let us join these two tables in our SELECT statement as follows −
SELECT ID, NAME, AGE, AMOUNT FROM CUSTOMERS, ORDERS WHERE CUSTOMERS.ID = ORDERS.CUSTOMER_ID OR SELECT A.ID, A.NAME, A.AGE, B.AMOUNT FROM CUSTOMERS A inner join ORDERS B on A.ID = B.Customer_ID
The above command will produce the following output.
ID NAME AGE AMOUNT 2 Khilan 25 1560.00 3 kaushik 23 1500.00 3 kaushik 23 3000.00 4 Chaitali 25 2060.00
It is noticeable that the join is performed in the WHERE clause. Several operators can be used to join tables, such as =, <, >, <>, <=, >=, !=, BETWEEN, LIKE, and NOT; they can all be used to join tables. However, the most common operator is the equal symbol.
MS SQL Server Join Types −
There are different types of joins available in MS SQL Server −
INNER JOIN − Returns rows when there is a match in both tables.
LEFT JOIN − Returns all rows from the left table, even if there are no matches in the right table.
RIGHT JOIN − Returns all rows from the right table, even if there are no matches in the left table.
FULL JOIN − Returns rows when there is a match in one of the tables.
SELF JOIN − This is used to join a table to itself as if the table were two tables, temporarily renaming at least one table in the MS SQL Server statement.
CARTESIAN JOIN − Returns the Cartesian product of the sets of records from the two or more joined tables.