18_左右连接练习

练习1.三表查询

create table threeTA(

id int,

name char(5)

);

insert into threeTA

values

(1,"面包"),

(2,"跑步机"),

(3,"键盘");

create table threeTB(

name char(5),

price int

);

insert into threeTB

values

("面包",5),

("跑步机",3000),

("键盘",100);

create table threeTC(

price int,

sort char(5)

);

insert into threeTC

values

(5,"低端"),

(3000,"高端"),

(100,"中端");

select * from threeTA;

+------+--------+

| id   | name   |

+------+--------+

|    1 | 面包   |

|    2 | 跑步机 |

|    3 | 键盘   |

+------+--------+

select * from threeTB;

+--------+-------+

| name   | price |

+--------+-------+

| 面包   |     5 |

| 跑步机 |  3000 |

| 键盘   |   100 |

+--------+-------+

select * from threeTC;

+-------+------+

| price | sort |

+-------+------+

|     5 | 低端 |

|  3000 | 高端 |

|   100 | 中端 |

+-------+------+

建表完成后，三表连查：

select id,threeTA.name,threeTB.price,sort from 

threeTA left join threeTB

on threeTA.name=threeTB.name

left join threeTC

on threeTB.price=threeTC.price;

+------+--------+-------+------+

| id   | name   | price | sort |

+------+--------+-------+------+

|    1 | 面包   |     5 | 低端 |

|    2 | 跑步机 |  3000 | 高端 |

|    3 | 键盘   |   100 | 中端 |

+------+--------+-------+------+