通过两个数据表来讲解多表查询
1. join
select * from a join b on a.id=b.id;
查询结果:
内连接查询数据结果是多表查询的交集
2.left join
select * from a leftjoin b on a.id=b.id;
查询结果:
左连接查询的结果集满足:
3.right join
select * from a right join b on a.id=b.id;
查询结果:
右连接查询的结果集满足:
4.union(全连接)
select * from a left join b on a.id=b.id union select * from a right join b on a.id=b.id;
查询结果:
全连接查询的结果集满足:
5.交叉连接
select * from a cross join b;
查询结果: