mysql 多表查询

mysql 多表查询

目录

• mysql 多表查询
  • 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)
  • 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
    • 练习
  • 子查询

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)

作 者：郭楷丰

出 处：https://www.cnblogs.com/guokaifeng/

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