What is the difference between left join, right join, inner join and full outer join?

Difference: left join returns all records in the left table that are equal to the join field in the right table; right join returns all records in the right table and records that are equal to the join field in the left table; inner join Only return rows with equal join fields in the two tables; full outer joins return all records in the left and right tables and records with equal join fields in the left and right tables.

left join (left join, left outer join): Returns all records in the left table that are equal to the join fields in the right table.

right join (right join, right outer join): Returns all records in the right table that are equal to the join fields in the left table.

inner join (equivalent join or inner join): Only return rows with equal join fields in the two tables.

full join (full outer join): Returns all records in the left and right tables that are equal to the join fields in the left and right tables.

For example:

A table

id Name　

1 Xiao Wang

2 Xiao Li

3

#Inner join: (Only matching rows from 2 tables can be displayed)

select a.name,b.job from A a  inner join B b on a.id=b.A_id

Only one record can be obtained:

小李　　老师

Left join: (The one on the left The table is not restricted)

select a.name,b.job from A a  left join B b on a.id=b.A_id

Three records:

　　小王　　null
 
　　小李　　老师
 
　　小刘　　null

Right join: (The table on the right is not restricted)

select a.name,b.job from A a  right join B b on a.id=b.A_id

Two Records:

　　小李　　老师
 
　　null　　程序员

Full external join: (no restrictions on the left and right tables)

select a.name,b.job from A a  full join B b on a.id=b.A_id

Four pieces of data

　　小王　　null
 
　　小李　　老师
 
　　小刘　　null
 
　　null　　程序员

Note: In SQL, outer joins include left join and right join, full outer join, and equal join (inner join), also called inner joins.

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