//本文使用的数据表格
//persons表中id_p为主键
//orders表中id_o为主键,id_p为外键参考persons表中的id_p
mysql> select * from persons;select * from orders; +------+----------+-----------+----------------+----------+ | id_p | lastname | firstname | address | city | +------+----------+-----------+----------------+----------+ | 1 | Adams | John | Oxford Street | London | | 2 | Bush | George | Fifth Avenue | New York | | 3 | Carter | Thomas | Changan Street | Beijing | +------+----------+-----------+----------------+----------+ 3 rows in set (0.05 sec) +------+---------+------+ | id_o | orderNo | id_p | +------+---------+------+ | 1 | 77895 | 3 | | 2 | 44678 | 3 | | 3 | 22456 | 1 | | 4 | 24562 | 1 | | 5 | 34764 | 65 | +------+---------+------+ 5 rows in set (0.05 sec)
1.内连接 [inner] join:
内连接也叫连接,或者自然连接:
//inner可删 mysql> select lastname,firstname,orderNo from persons inner join orders on persons.id_p=orders.id_p order by lastname; +----------+-----------+---------+ | lastname | firstname | orderNo | +----------+-----------+---------+ | Adams | John | 24562 | | Adams | John | 22456 | | Carter | Thomas | 77895 | | Carter | Thomas | 44678 | +----------+-----------+---------+ //使用where达到同样的效果 mysql> select lastname,firstname,orderNo from persons,orders where persons.id_p=orders.id_p; +----------+-----------+---------+ | lastname | firstname | orderNo | +----------+-----------+---------+ | Carter | Thomas | 77895 | | Carter | Thomas | 44678 | | Adams | John | 22456 | | Adams | John | 24562 | +----------+-----------+---------+
2.左连接 left [outer] join:
先看左连接的语句与查询结果
//outer可删去 mysql> select lastname,firstname,orderNo from persons left join orders on persons.id_p=orders.id_p order by lastname; +----------+-----------+---------+ | lastname | firstname | orderNo | +----------+-----------+---------+ | Adams | John | 24562 | | Adams | John | 22456 | | Bush | George | NULL | | Carter | Thomas | 77895 | | Carter | Thomas | 44678 | +----------+-----------+---------+
通过这个结果我们可以发现lastname为Bush没有orderNo,而lastname,firstname属性是来自persons表,orderNo属性是来自orders表
3.右连接 right [outer] join:
//outer可删去 mysql> select lastname,firstname,orderNo from persons right outer join orders on persons.id_p=orders.id_p order by lastname; +----------+-----------+---------+ | lastname | firstname | orderNo | +----------+-----------+---------+ | NULL | NULL | 34764 | | Adams | John | 22456 | | Adams | John | 24562 | | Carter | Thomas | 77895 | | Carter | Thomas | 44678 | +----------+-----------+---------+