Joins

The SQL 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

CUSTOMERS 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

ORDERS Table

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

Join these two tables in our SELECT statement as shown below.

SELECT ID, NAME, AGE, AMOUNT

   FROM CUSTOMERS, ORDERS

   WHERE CUSTOMERS.ID = ORDERS.CUSTOMER_ID;

Output:

ID	NAME	AGE	AMOUNT
3	kaushik	23	3000
3	kaushik	23	1500
2	Khilan	25	1560
4	Chaitali	25	2060

Basic SQL Join Types

There are four basic types of SQL joins: inner, left, right, and full. 

Inner Join

The most important and frequently used of the joins is the INNER JOIN. They are also referred to as an EQUIJOIN. The INNER JOIN creates a new result table by combining column values of two tables (table A and table B) based upon the join-predicate. The query compares each row of table A with each row of table B to find all pairs of rows which satisfy the join-predicate. When the join-predicate is satisfied, column values for each matched pair of rows of A and B are combined into a result row.

The basic syntax of the INNER JOIN is as follows.

SELECT table1.column1, table2.column2...

FROM table1

INNER JOIN table2

ON table1.common_field = table2.common_field;

Example:

SELECT  ID, NAME, AMOUNT, DATE

   FROM CUSTOMERS

   INNER JOIN ORDERS

   ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;

Output:

ID	NAME	AMOUNT	DATE
3	kaushik	3000	2009-10-08 00:00:00
3	kaushik	1500	2009-10-08 00:00:00
2	Khilan	1560	2009-11-20 00:00:00
4	Chaitali	2060	2008-05-20 00:00:00

Left Join

The SQL LEFT JOIN returns all rows from the left table, even if there are no matches in the right table. This means that a left join returns all the values from the left table, plus matched values from the right table or NULL in case of no matching join predicate.

The basic syntax of a LEFT JOIN is as follows.

SELECT table1.column1, table2.column2...

FROM table1

LEFT JOIN table2

ON table1.common_field = table2.common_field;

Example:

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

Right Join

The SQL RIGHT JOIN returns all rows from the right table, even if there are no matches in the left table. This means that a right join returns all the values from the right table, plus matched values from the left table or NULL in case of no matching join predicate.

The basic syntax of a RIGHT JOIN is as follow.

SELECT table1.column1, table2.column2...

FROM table1

RIGHT JOIN table2

ON table1.common_field = table2.common_field;

Example:

SELECT  ID, NAME, AMOUNT, DATE

   FROM CUSTOMERS

   RIGHT JOIN ORDERS

   ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;

Output:

ID	NAME	AMOUNT	DATE
3	kaushik	3000	2009-10-08 00:00:00
3	kaushik	1500	2009-10-08 00:00:00
2	Khilan	1560	2009-11-20 00:00:00
4	Chaitali	2060	2008-05-20 00:00:00

Full Join

The SQL FULL JOIN combines the results of both left and right outer joins. The joined table will contain all records from both the tables and fill in NULLs for missing matches on either side.

The basic syntax of a FULL JOIN is as follows

SELECT table1.column1, table2.column2...

FROM table1

FULL JOIN table2

ON table1.common_field = table2.common_field;

Example:

SELECT  ID, NAME, AMOUNT, DATE

   FROM CUSTOMERS

   FULL 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
3	kaushik	3000	2009-10-08 00:00:00
3	kaushik	1500	2009-10-08 00:00:00
2	Khilan	1560	2009-11-20 00:00:00
4	Chaitali	2060	2008-05-20 00:00:00