SQL之内连接与外连接

内连接:

内连接即等值连接

select a.name,b.age from a inner join b on a.id = b.id;

select a.name,b.age from a,b where a.id = b.id;

左连接:

在等值连接的基础上加上主表(左表)中未匹配的记录

select a.name,b.age from a left outer join b on a.id=b.id;

select a.name,b.age,c.sex from a left outer join b on a.id = b.id  left outer join on a.id = c.id;

右连接:

在等值连接的基础上加上主表(右表)中未匹配的记录

select a.name,b.age from a right outer join b on a.id=b.id;

select a.name,b.age,c.sex from a right outer b on a.id = b.id  right outer join c on b.id = c.id;

注意: 可以将a left outer join b on a.id = b.id看成是一张C表 on 后面的条件无论是否为真,都返回a表的全部记录