An SQL JOIN clause combines rows from two or more tables. It creates a set of rows in a temporary table.
A JOIN works on two or more tables if they have at least one common field and have a relationship between them.
JOIN keeps the base tables (structure and data) unchanged.
The are two types of SQL JOINS - EQUI JOIN and NON EQUI JOIN
1) SQL EQUI JOIN :
The SQL EQUI JOIN is a simple SQL join uses the equal sign(=) as the comparison operator for the condition. It has two types - SQL Outer join and SQL Inner join.
2) SQL NON EQUI JOIN :
The SQL NON EQUI JOIN is a join uses comparison operator other than the equal sign like >, <, >=, <= with the condition.
The SQL EQUI JOIN can be classified into two types - INNER JOIN and OUTER JOIN
1. SQL INNER JOIN
This type of EQUI JOIN returns all rows from tables where the key record of one table is equal to the key records of another table.
2. SQL OUTER JOIN
This type of EQUI JOIN returns all rows from one table and only those rows from the secondary table where the joined condition is satisfying i.e. the columns are equal in both tables.
In order to perform a JOIN query, the required information we need are:
a) The name of the tables
b) Name of the columns of two or more tables, based on which a condition will perform.
FROM table1 join_type table2 [ON (join_condition)]
|table1, table2||Tables participating in joining.|
|join_type||Type of the join.|
|join_condition||Some condition. This is optional.|
Sample table : company
Sample table : foods
To join two tables 'company' and 'foods', the following SQL statement can be used :
SELECT company.company_id,company.company_name, foods.item_id,foods.item_name FROM company,foods;
COMPAN COMPANY_NAME ITEM_ID ITEM_NAME ------ ------------------------- -------- --------------- 18 Order All 1 Chex Mix 18 Order All 6 Cheez-It 18 Order All 2 BN Biscuit 18 Order All 3 Mighty Munch 18 Order All 4 Pot Rice 18 Order All 5 Jaffa Cakes 18 Order All 7 Salt n Shake 15 Jack Hill Ltd 1 Chex Mix 15 Jack Hill Ltd 6 Cheez-It 15 Jack Hill Ltd 2 BN Biscuit 15 Jack Hill Ltd 3 Mighty Munch 15 Jack Hill Ltd 4 Pot Rice 15 Jack Hill Ltd 5 Jaffa Cakes 15 Jack Hill Ltd 7 Salt n Shake 16 Akas Foods 1 Chex Mix 16 Akas Foods 6 Cheez-It 16 Akas Foods 2 BN Biscuit 16 Akas Foods 3 Mighty Munch 16 Akas Foods 4 Pot Rice 16 Akas Foods 5 Jaffa Cakes 16 Akas Foods 7 Salt n Shake ......... ......... .........
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