mysql joins

JOINS IN MYSQL

by

Anjali G

INTRODUCTION

Join help us to retrieve data from multiple tables in a single query.

We can use joins in select, update and delete statements.

Types of join:

1) Inner join

2) Cross join

3) Left join

4)Right join

5)Self join

INNER JOIN

This join returns rows when there is at least one match in both the tables.

Criteria's before using inner join:

1) first specify the main table it appears in the from clause.

2) Specify the table that want to join with the main table. This is in the inner join clause.

3) Need to specify the join condition or join predicate. The join condition appears after the keyword ON of  the INNER JOIN clause. The join condition is the rule for matching rows between the main table and the other tables.

Eg: SELECT xx, xxtable.id, yytable.id, yy

FROM xxtable

INNER JOIN yytable

ON xxtable.id=yytable.id;

SELECT * FROM table1 t1 INNER JOIN table 2 t2 ON t1.col1=t2.col1

CROSS JOIN

This join is a Cartesian join that does not necessitate any condition to join.

Result set contains record that are multiplication of record number from both the tables.

Also known as Cartesian product join

Eg: select * from table 1 cross join table 2 ;

LEFT JOIN

LEFT JOIN produces a set of records which matches every entry in the left table regardless of any matching entry in the right table

If there is a row in A that matches the WHERE clause, but there is no row in B that matches the ON condition, an extra B row is generated with all columns set to NULL.

RIGHT JOIN

This join returns all the rows from the right table in conjunction with the matching rows from the left table. If there are no columns matching in the left table, it returns NULL values.

Example: SELECT *FROM employeeRIGHT OUTER JOIN employee_details ON

employee_details.employee_id = employee.employee_id

SELF JOIN

A self join is a join in which a table is joined with itselfIn this each row of the table is combined with itself and with every other row of the table.

Example:

SELECT e1.name, e2.name,e1.emailFROM employees AS e1, employees AS e2WHERE e1.email = e2.email 

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