The difference between left join and right join, inner join and full outer join in SQL

left join (left join): Returns all records including all records in the left table and records with the same connection field in the right table.

right join (right join): Returns all records including all records in the right table and records with equal connection fields in the left table.

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

Full join (full outer join): Return all records in the left and right tables and records with the same connection fields in the left and right tables.

Table A:

id	name
1	Xiao Wang
2	Xiao Li
3	Xiao Liu

 

Table B:

id	A_id	job
1	2	teacher
2	4	programmer

 

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

search result:

name	job
Xiao Li	teacher

 

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

search result:

name	job
Xiao Wang	null
Xiao Li	teacher
Xiao Liu	null

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

search result:

name	job
Xiao Li	teacher
null	programmer

全外连接：select a.name,b.joib from A a full join B b on a.id=b.A_id;

search result:

name	job
Xiao Wang	null
Xiao Li	teacher
Xiao Liu	null
null	programmer