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, (a) CUSTOMERS table is as follows :-
If your Database does not support FULL JOIN like MySQL does not support FULL JOIN, then you can use UNION
ALL clause to combine two JOINS as follows :-
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID
UNION ALL
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
RIGHT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID
SELF JOIN
The SQL SELF JOIN is used to join a table to itself as if the table were two tables, temporarily renaming at least one table in the SQL statement.
Syntax :-
The basic syntax of SELF JOIN is as follows :-
SELECT a.column_name, b.column_name...
FROM table1 a, table1 b
WHERE a.common_filed = b.common_field;
Here, WHERE clause could be any given expression based on your requirement.
Example :-
Consider the following two tables, (a) CUSTOMERS table is as follows :-
Consider the following two tables, (a) CUSTOMERS table is as follows :-
(b) Another table is ORDERS as follows :-
Now, 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;
This would produce the following result :-
Here, 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.
SQL Join Types :-
There are different types of joins available in SQL :-
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 :- is used to join a table to itself as if the table were two tables, temporarily renaming at least one table in the SQL statement.
CARTESIAN JOIN :- returns the Cartesian product of the sets of records from the two or more joined tables.
INNER JOIN
The most frequently used and important 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 (table1 and table2) based upon the join-predicate. The query compares each row of table1 with each row of table2 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.
Syntax :-
The basic syntax of INNER JOIN is as follows :-
SELECT table1.column1, table2.column2...
FROM table1
INNER JOIN table2
ON table1.common_filed = table2.common_field;
Example :-
Consider the following two tables, (a) CUSTOMERS table is as follows :-
(b) Another table is ORDERS as follows :-
Now, let us join these two tables using INNER JOIN as follows :-
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
INNER JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;
This would produce the following result :-
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 if the ON clause matches 0 (zero) records in right table, the join will still return a row in the result, but with NULL in each column from 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.
Syntax :-
The basic syntax of LEFT JOIN is as follows :-
SELECT table1.column1, table2.column2...
FROM table1
LEFT JOIN table2
ON table1.common_filed = table2.common_field;
Here given condition could be any given expression based on your requirement.
Example :-
Consider the following two tables, (a) CUSTOMERS table is as follows :-
(b) Another table is ORDERS as follows :-
Now, let us join these two tables using LEFT JOIN as follows :-
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;
This would produce the following result :-
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 if the ON clause matches 0 (zero) records in left table, the join will still return a row in the result , but with NULL in each column from 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.
Syntax :-
The basic syntax of RIGHT JOIN is as follows :-
SELECT table1.column1, table2.column2...
FROM table1
RIGHT JOIN table2
ON table1.common_filed = table2.common_field;
Example :-
Consider the following two tables, (a) CUSTOMERS table is as follows :-
(b) Another table is ORDERS as follows :-
Now, let us join these two tables using RIGHT JOIN as follows :-
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
RIGHT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;
This would produce the following result :-
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 tables, and fill in NULLs for missing matches on either side.
Syntax :-
The basic syntax of FULL JOIN is as follows :-
SELECT table1.column1, table2.column2...
FROM table1
FULL JOIN table2
ON table1.common_filed = table2.common_field;
Here given condition could be any given expression based on your requirement.
Example :-
Consider the following two tables, (a) CUSTOMERS table is as follows :-
(b) Another table is ORDERS as follows :-
Now, let us join these two tables using FULL JOIN as follows :-
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
FULL JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;
This would produce the following result :-
ALL clause to combine two JOINS as follows :-
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID
UNION ALL
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
RIGHT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID
SELF JOIN
The SQL SELF JOIN is used to join a table to itself as if the table were two tables, temporarily renaming at least one table in the SQL statement.
Syntax :-
The basic syntax of SELF JOIN is as follows :-
SELECT a.column_name, b.column_name...
FROM table1 a, table1 b
WHERE a.common_filed = b.common_field;
Here, WHERE clause could be any given expression based on your requirement.
Example :-
Consider the following two tables, (a) CUSTOMERS table is as follows :-
Now, let us join this table using SELF JOIN as follows :-
SELECT a.ID, b.NAME, a.SALARY
FROM CUSTOMERS a, CUSTOMERS b
WHERE a.SALARY < b.SALARY;
This would produce the following result :-
CARTESIAN JOIN
The CARTESIAN JOIN or CROSS JOIN returns the cartesian product of the sets of records from the two or more joined tables. Thus, it equates to an inner join where the join-condition always evaluates to True or where the joincondition is absent from the statement.
Syntax :-
The basic syntax of INNER JOIN is as follows :-
SELECT table1.column1, table2.column2...
FROM table1, table2 [, table3 ]
Example :-
Consider the following two tables, (a) CUSTOMERS table is as follows :-
(b) Another table is ORDERS as follows :-
Now, let us join these two tables using INNER JOIN as follows :-
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS, ORDERS;
This would produce the following result :-
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