mysql 多表查询
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2023-04-05 16:52:33
mysql 多表查询 [TOC] 数据准备 连表查 交叉连接 不适用任何匹配条件,生成笛卡尔积 select from 表1,表2; 内链接 (常用) 只连接匹配的行 select from staff inner join department on 条件(表1.字段=表2.字段) 小结: 找两张 ......
目录
mysql 多表查询
数据准备
#建表 create table department( id int, name varchar(20) ); create table staff( id int primary key auto_increment, name varchar(20), sex enum('male','female') not null default 'male', age int, dep_id int ); #插入数据 insert into department values (200,'挖矿小分队'), (201,'人力资源'), (202,'销售'), (203,'运营'); insert into staff(name,sex,age,dep_id) values ('程咬金','male',38,200), ('露娜','female',26,201), ('李白','male',38,201), ('王昭君','female',28,202), ('典韦','male',118,200), ('小乔','female',16,204) ; #查看表结构和数据 mysql> desc department; +-------+-------------+------+-----+---------+-------+ | field | type | null | key | default | extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | yes | | null | | | name | varchar(20) | yes | | null | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.06 sec) mysql> desc staff; +--------+-----------------------+------+-----+---------+----------------+ | field | type | null | key | default | extra | +--------+-----------------------+------+-----+---------+----------------+ | id | int(11) | no | pri | null | auto_increment | | name | varchar(20) | yes | | null | | | sex | enum('male','female') | no | | male | | | age | int(11) | yes | | null | | | dep_id | int(11) | yes | | null | | +--------+-----------------------+------+-----+---------+----------------+ 5 rows in set (0.04 sec) #表department与staff mysql> select * from department; +------+-----------------+ | id | name | +------+-----------------+ | 200 | 挖矿小分队 | | 201 | 人力资源 | | 202 | 销售 | | 203 | 运营 | +------+-----------------+ 4 rows in set (0.00 sec) mysql> select * from staff; +----+-----------+--------+------+--------+ | id | name | sex | age | dep_id | +----+-----------+--------+------+--------+ | 1 | 程咬金 | male | 38 | 200 | | 2 | 露娜 | female | 26 | 201 | | 3 | 李白 | male | 38 | 201 | | 4 | 王昭君 | female | 28 | 202 | | 5 | 典韦 | male | 118 | 200 | | 6 | 小乔 | female | 16 | 204 | +----+-----------+--------+------+--------+ 6 rows in set (0.00 sec)
连表查
交叉连接 不适用任何匹配条件,生成笛卡尔积
select * from 表1,表2;
内链接 (常用) 只连接匹配的行
select * from staff inner join department on 条件(表1.字段=表2.字段)
小结: 找两张表共有的部分,利用条件从笛卡尔积结果中筛选出了正确的结果
外连接
左外连接(常用) 优先显示左表全部记录 left join
select * from staff left join department on 条件(表1.字段=表2.字段)
右外链接 优先显示右表全部记录 right join
select * from staff right join department on 条件(表1.字段=表2.字段)
全外连接 显示左右两个表全部记录
- 查询语句 (mysql没有full join,可以有左外连接+右外连接来实现全外连接)
select * from 表1 left join 表2 on 条件((表1.字段=表2.字段)) union select * from 表1 right join 表2 on 条件((表1.字段=表2.字段));
#全外连接:在内连接的基础上增加左边有右边没有的和右边有左边没有的结果 #注意:mysql不支持全外连接 full join #强调:mysql可以使用此种方式间接实现全外连接 #注意 union与union all的区别:union会去掉相同的纪录
练习
1.找挖矿小分队的所有员工的信息 #答: mysql> select * from staff inner join department on department.id=staff.dep_id where department.name='挖矿小分队'; +----+-----------+------+------+--------+------+-----------------+ | id | name | sex | age | dep_id | id | name | +----+-----------+------+------+--------+------+-----------------+ | 1 | 程咬金 | male | 38 | 200 | 200 | 挖矿小分队 | | 5 | 典韦 | male | 118 | 200 | 200 | 挖矿小分队 | +----+-----------+------+------+--------+------+-----------------+ 2 rows in set (0.00 sec) 2.查找人力资源所有的员工名字 #答: (名字太长可以起别名) mysql> select staff.name from staff inner join department as dep on dep.id=staff.dep_id where dep.name='人力资源'; +--------+ | name | +--------+ | 露娜 | | 李白 | +--------+ 2 rows in set (0.00 sec) 3.找出年龄大于38的员工的姓名,及其所在的部门名称 #答: mysql> select staff.name,dep.name from staff inner join department as dep on dep.id=staff.dep_id where age>38; +--------+-----------------+ | name | name | +--------+-----------------+ | 典韦 | 挖矿小分队 | +--------+-----------------+ 1 row in set (0.00 sec) 4.以内连接的方式查询 staff 和 department表,并且以age字段的升序方式显示 答: mysql> select * from staff inner join department as dep on dep.id=staff.dep_id order by age; +----+-----------+--------+------+--------+------+-----------------+ | id | name | sex | age | dep_id | id | name | +----+-----------+--------+------+--------+------+-----------------+ | 2 | 露娜 | female | 26 | 201 | 201 | 人力资源 | | 4 | 王昭君 | female | 28 | 202 | 202 | 销售 | | 1 | 程咬金 | male | 38 | 200 | 200 | 挖矿小分队 | | 3 | 李白 | male | 38 | 201 | 201 | 人力资源 | | 5 | 典韦 | male | 118 | 200 | 200 | 挖矿小分队 | +----+-----------+--------+------+--------+------+-----------------+ 5 rows in set (0.05 sec) 5.找到部门为 挖矿小分队 和 人力资源 的所有员工的名字 #答: mysql> select staff.name from staff inner join department as dep on dep.id = staff.dep_id where dep.name in ('挖矿小分队','人力资源'); +-----------+ | name | +-----------+ | 程咬金 | | 露娜 | | 李白 | | 典韦 | +-----------+ 4 rows in set (0.00 sec)
子查询
#1:子查询是将一个查询语句嵌套在另一个查询语句中。 #2:内层查询语句的查询结果,可以为外层查询语句提供查询条件。 #3:子查询中可以包含:in、not in、any、all、exists 和 not exists等关键字 #4:还可以包含比较运算符:= 、 !=、> 、<等 #5: 多用连表查,因为连表查询比子查询效率高 1.用子查询 找到部门是销售的所有员工的姓名 #解题思路 ①先找department表部门为销售的部门的id mysql> select id from department where name = '销售'; +------+ | id | +------+ | 202 | +------+ 1 row in set (0.00 sec) ②再找staff表中部门dep_id = 202 mysql> select name from staff where dep_id = 202; +-----------+ | name | +-----------+ | 王昭君 | +-----------+ 1 row in set (0.00 sec) ③字表查 mysql> select name from staff where dep_id =(select id from department where name = '销售'); +-----------+ | name | +-----------+ | 王昭君 | +-----------+ 1 row in set (0.00 sec) 2.用子查询 找到部门为 销售 和 人力资源 的所有员工的名字 ①先找department表部门为销售和人力资源的部门的id mysql> select id from department where name = '销售' or name = '人力资源'; +------+ | id | +------+ | 201 | | 202 | +------+ 2 rows in set (0.00 sec) ②子查询 mysql> select name from staff where dep_id in (select id from department where name = '销售' or name = '人力资源'); +-----------+ | name | +-----------+ | 露娜 | | 李白 | | 王昭君 | +-----------+ 3 rows in set (0.00 sec)
1. 带in关键字的子查询 ①查询平均年龄在28岁以上的部门名 select id,name from department where id in (select dep_id from staff group by dep_id having avg(age) > 28); #结果 +------+-----------------+ | id | name | +------+-----------------+ | 200 | 挖矿小分队 | | 201 | 人力资源 | +------+-----------------+ 2 rows in set (0.00 sec) ②查看部门是挖矿小分队员工姓名 select name from staff where dep_id in (select id from department where name='挖矿小分队'); #结果 +-----------+ | name | +-----------+ | 程咬金 | | 典韦 | +-----------+ 2 rows in set (0.00 sec) ③查看不足1人的部门名(子查询得到的是有人的部门id) select name from department where id not in (select distinct dep_id from staff); #结果 +--------+ | name | +--------+ | 运营 | +--------+ 1 row in set (0.02 sec) 2. 带比较运算符的子查询 #比较运算符:=、!=、>、>=、<、<=、<> ①查询大于所有人平均年龄的员工名与年龄 mysql> select name,age from staff where age > (select avg(age) from staff); +--------+------+ | name | age | +--------+------+ | 典韦 | 118 | +--------+------+ 1 row in set (0.00 sec) ②查询大于部门内平均年龄的员工名、年龄 select t1.name,t1.age from staff t1 inner join (select dep_id,avg(age) avg_age from staff group by dep_id) t2 on t1.dep_id = t2.dep_id where t1.age > t2.avg_age; #结果 +--------+------+ | name | age | +--------+------+ | 李白 | 38 | | 典韦 | 118 | +--------+------+ 2 rows in set (0.04 sec) 3. 带exists关键字的子查询 exists关字键字表示存在。在使用exists关键字时,内层查询语句不返回查询的记录。 而是返回一个真假值。true或false 当返回true时,外层查询语句将进行查询;当返回值为false时,外层查询语句不进行查询 #department表中存在dept_id=203,ture select * from staff where exists (select id from department where id=200); #结果 +----+-----------+--------+------+--------+ | id | name | sex | age | dep_id | +----+-----------+--------+------+--------+ | 1 | 程咬金 | male | 38 | 200 | | 2 | 露娜 | female | 26 | 201 | | 3 | 李白 | male | 38 | 201 | | 4 | 王昭君 | female | 28 | 202 | | 5 | 典韦 | male | 118 | 200 | | 6 | 小乔 | female | 16 | 204 | +----+-----------+--------+------+--------+ 6 rows in set (0.00 sec) #department表中存在dept_id=205,false mysql> select * from staff where exists (select id from department where id=204); empty set (0.00 sec)
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