SQL 语句 连接查询，子查询

关键词 JOIN，IN ，ALL ，主键（primary key）外键(foreign key)

连接查询 select a,b from db_a a inner join db_b b on a.id=b.y where a.x=‘xx’

 也可以不用join把连接两个db的条件放到where语句中用AND连接

 

子查询 select a from db_a where a > ALL(select c from db_b where b='xx')

ALL()括号里面的是c的集合

 

select a from db_a where a IN (select c from db_b where b='xx') 

IN() 括号里C的集合