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查询

连接查询

也叫多表查询，常用于查询表头来自于多张表

通过不同连接方式把多张表临时组成一张表 在临时的表里查找符合条件的数据

内连接查询

语法：

select 表头名 from 表名a inner join 表名b;        迪卡尔积

                                5                *        5                       25

mysql> select * from departments;
......
......
8 rows in set (0.01 sec)

mysql> select * from employees;
......
......
133 rows in set (0.00 sec)

mysql> select * from employees inner join departments;
......
......
1064 rows in set (0.00 sec)

语法：

select 表头名 from 表名a inner join 表名b on 连接条件 where | group by | having | order by | limit;（符合连接条件的行 才会出现在新表）

等值连接

连接的表里有存储相同数据的表头才能使用相等判断做连接条件

mysql> select * from employees inner join departments on employees.dept_id = departments.dept_id;
......
......
133 rows in set (0.01 sec)

mysql> select * from employees inner join departments on employees.dept_id = departments.dept_id limit 1;
+-------------+--------+------------+------------+------------------+--------------+---------+---------+-----------+
| employee_id | name   | hire_date  | birth_date | email            | phone_number | dept_id | dept_id | dept_name |
+-------------+--------+------------+------------+------------------+--------------+---------+---------+-----------+
|           1 | 梁伟   | 2018-06-21 | 1971-08-19 | [email protected] | 13591491431  |       1 |       1 | 人事部    |
+-------------+--------+------------+------------+------------------+--------------+---------+---------+-----------+
1 row in set (0.00 sec)

非等值连接

不使用相等判断做连接条件

表里没有存储相同数据的表头时使用

外链接

左连接查询

左表表头记录全部显示

右表表头只显示与条件匹配的记录，右表比左表少的记录使用NULL匹配

select 表头名 from 表名a left join 表名b on 连接条件 where | group by | having | order by | limit;

环境准备：
# 向departments表里添加3个部门:小卖部 行政部 公关部
mysql> insert into departments(dept_name) values("小卖部"),("行政部"),("公关部");

# 查询部门信息
mysql> select * from departments;
+---------+-----------+
| dept_id | dept_name |
+---------+-----------+
|       1 | 人事部    |
|       2 | 财务部    |
|       3 | 运维部    |
|       4 | 开发部    |
|       5 | 测试部    |
|       6 | 市场部    |
|       7 | 销售部    |
|       8 | 法务部    |
|       9 | 小卖部    |
|      10 | 行政部    |
|      11 | 公关部    |
+---------+-----------+
11 rows in set (0.00 sec)

# 输出没有员工的部门名
mysql> select departments.dept_id,departments.dept_name,employees.name from departments left join employees on employees.dept_id = departments.dept_id;
+---------+-----------+-----------+
| dept_id | dept_name | name      |
+---------+-----------+-----------+
|       1 | 人事部    | 梁伟      |
|       1 | 人事部    | 郭岩      |

......
......
|       8 | 法务部    | 刘倩      |
|       8 | 法务部    | 杨金凤    |
|       9 | 小卖部    | NULL      |
|      10 | 行政部    | NULL      |
|      11 | 公关部    | NULL      |
+---------+-----------+-----------+
136 rows in set (0.00 sec)

# 输出与筛选条件匹配的行
mysql> select departments.dept_id,departments.dept_name,employees.name from departments left join employees on employees.dept_id = departments.dept_id where name is null;
+---------+-----------+------+
| dept_id | dept_name | name |
+---------+-----------+------+
|       9 | 小卖部    | NULL |
|      10 | 行政部    | NULL |
|      11 | 公关部    | NULL |
+---------+-----------+------+
3 rows in set (0.00 sec)

# 仅显示departments表中dept_name表头
mysql> select departments.dept_name from departments left join employees on employees.dept_id = departments.dept_id where name is null; +-----------+
| dept_name |
+-----------+
| 小卖部    |
| 行政部    |
| 公关部    |
+-----------+
3 rows in set (0.00 sec)

#验证  插入数据
mysql> insert into employees(name,dept_id) values("bob",9);

mysql> select departments.dept_name from departments left join employees on employees.dept_id = departments.dept_id where name is null;
+-----------+
| dept_name |
+-----------+
| 行政部    |
| 公关部    |
+-----------+
2 rows in set (0.00 sec)

右连接查询

右表表头记录全显示

左表表头只显示与条件匹配的记录，左表比右表少的记录使用NULL匹配

select 表头名 from 表名a right join 表名b on 连接条件 where | group by | having | order by | limit;

# 环境准备
# 向employees表中添加3个员工 只给name表头赋值
mysql> insert into employees(name) values("john"),("alice"),("jerry");
mysql> select * from employees where name in ("john","alice","jerry");
+-------------+-------+-----------+------------+-------+--------------+---------+
| employee_id | name  | hire_date | birth_date | email | phone_number | dept_id |
+-------------+-------+-----------+------------+-------+--------------+---------+
|         135 | john  | NULL      | NULL       | NULL  | NULL         |    NULL |
|         136 | alice | NULL      | NULL       | NULL  | NULL         |    NULL |
|         137 | jerry | NULL      | NULL       | NULL  | NULL         |    NULL |
+-------------+-------+-----------+------------+-------+--------------+---------+
3 rows in set (0.00 sec)

# 显示没有部门的员工名
mysql> select name,dept_name from departments right join employees on departments.dept_id = employees.dept_id where dept_name is null;
+-------+-----------+
| name  | dept_name |
+-------+-----------+
| john  | NULL      |
| alice | NULL      |
| jerry | NULL      |
+-------+-----------+
3 rows in set (0.00 sec)

mysql> select name from departments right join employees on departments.dept_id = employees.dept_id where dept_name is null;
+-------+
| name  |
+-------+
| john  |
| alice |
| jerry |
+-------+
3 rows in set (0.00 sec)

# 验证 修改john的部门
mysql> update employees set dept_id=11 where name="john";
mysql> select name from departments right join employees on departments.dept_id = employees.dept_id where dept_name is null;
+-------+
| name  |
+-------+
| alice |
| jerry |
+-------+
2 rows in set (0.00 sec)

全连接查询

一起输出多个select命令的查询结果

多个select命令查询的表头个数要相同

(select 命令) union (select 命令) ....                去重

(select 命令) union all (select 命令) ....           不去重

嵌套查询

select查询命令里 包含 select查询命令

包含的select查询命令要放在 ( ) 里

包含的select查询命令可以出现的位置：

where 命令之后

把查询结果作为查询条件使用

select 表头名 from 库.表 where 表头名 筛选条件 (select查询命令);

# 查询运维部所有员工信息
mysql> select dept_id from departments where dept_name="运维部";
+---------+
| dept_id |
+---------+
|       3 |
+---------+
1 row in set (0.00 sec)

mysql> select * from employees where dept_id=3;
+-------------+-----------+------------+------------+--------------------+--------------+---------+
| employee_id | name      | hire_date  | birth_date | email              | phone_number | dept_id |
+-------------+-----------+------------+------------+--------------------+--------------+---------+
|          14 | 廖娜      | 2012-05-20 | 1982-06-22 | [email protected]  | 15827928192  |       3 |
|          15 | 窦红梅    | 2018-03-16 | 1971-09-09 | [email protected] | 15004739483  |       3 |
|          16 | 聂想      | 2018-09-09 | 1999-06-05 | [email protected]   | 15501892446  |       3 |
|          17 | 陈阳      | 2004-09-16 | 1991-04-10 | [email protected]   | 15565662056  |       3 |
|          18 | 戴璐      | 2001-11-30 | 1975-05-16 | [email protected]      | 13465236095  |       3 |
|          19 | 陈斌      | 2019-07-04 | 2000-01-22 | [email protected] | 13621656037  |       3 |
+-------------+-----------+------------+------------+--------------------+--------------+---------+
6 rows in set (0.00 sec)

mysql> select name,dept_id from employees where dept_id=(select dept_id from departments where dept_name="运维部");
+-----------+---------+
| name      | dept_id |
+-----------+---------+
| 廖娜      |       3 |
| 窦红梅    |       3 |
| 聂想      |       3 |
| 陈阳      |       3 |
| 戴璐      |       3 |
| 陈斌      |       3 |
+-----------+---------+
6 rows in set (0.00 sec)

# 查询人事部2018年12月所有员工工资

# 查看人事部的部门id
mysql> select dept_id from departments where dept_name="人事部";
+---------+
| dept_id |
+---------+
|       1 |
+---------+
1 row in set (0.00 sec)

# 查找employees表里 人事部的员工id 
mysql> select employee_id from employees where dept_id = 1;                                               +-------------+
| employee_id |
+-------------+
|           1 |
|           2 |
|           3 |
|           4 |
|           5 |
|           6 |
|           7 |
|           8 |
+-------------+
8 rows in set (0.00 sec)

# 查询人事部2018年12月所有员工工资 
mysql> select employee_id,basic,bonus from salary where year(date)=2018 and month(date)=12 and employee_id in (select employee_id from employees where dept_id = (select dept_id from departments where dept_name=" 人事部"));
+-------------+-------+-------+
| employee_id | basic | bonus |
+-------------+-------+-------+
|           1 | 17016 |  7000 |
|           2 | 20662 |  9000 |
|           3 |  9724 |  8000 |
|           4 | 17016 |  2000 |
|           5 | 17016 |  3000 |
|           6 | 17016 |  1000 |
|           7 | 23093 |  4000 |
|           8 | 23093 |  2000 |
+-------------+-------+-------+
8 rows in set (0.00 sec)

# 查询人事部和财务部员工信息

#  查看人事部和财务部的 部门id
mysql> select dept_id from departments where dept_name="人事部" or dept_name="财务部";
+---------+
| dept_id |
+---------+
|       1 |
|       2 |
+---------+
2 rows in set (0.00 sec)

# 查询人事部和财务部员工信息
mysql> select name,dept_id from employees where dept_id in (select dept_id from departments where dept_name="人事部" or dept_name="财务部")
    -> ;
+-----------+---------+
| name      | dept_id |
+-----------+---------+
| 梁伟      |       1 |
| 郭岩      |       1 |
| 李玉英    |       1 |
| 张健      |       1 |
| 郑静      |       1 |
| 牛建军    |       1 |
| 刘斌      |       1 |
| 汪云      |       1 |
| 张建平    |       2 |
| 郭娟      |       2 |
| 郭兰英    |       2 |
| 王英      |       2 |
| 王楠      |       2 |
+-----------+---------+
13 rows in set (0.00 sec)

# 查询2018年12月所有比100号员工基本工资高的工资信息

# 把100号员工的基本工资查出来
mysql> select basic from salary where year(date)=2018 and month(date)=12 and employee_id=100; +-------+
| basic |
+-------+
| 14585 |
+-------+
1 row in set (0.00 sec)

# 查看比100号员工工资高的工资信息
mysql> select basic,employee_id from salary where year(date)=2018 and month(date)=12 and basic > (select basic from salary where year(date)=2018 and month(date)=12 and employee_id=100);
......
......
65 rows in set (0.00 sec)

having 命令之后

# 查询部门员工总人数比开发部总人数少 的 部门名称和人数

# 统计开发部员工总人数
mysql> select count(name) from employees where dept_id=(select dept_id from departments where dept_name=" 开发部");
+-------------+
| count(name) |
+-------------+
|          55 |
+-------------+
1 row in set (0.00 sec)

# 统计每个部门总人数
mysql> select dept_id,count(name) from employees group by dept_id;                                        +---------+-------------+
| dept_id | count(name) |
+---------+-------------+
|    NULL |           2 |
|       1 |           8 |
|       2 |           5 |
|       3 |           6 |
|       4 |          55 |
|       5 |          12 |
|       6 |           9 |
|       7 |          35 |
|       8 |           3 |
|       9 |           1 |
|      11 |           1 |
+---------+-------------+
11 rows in set (0.00 sec)

# 输出总人数比开发部总人数少的部门名及总人数 
mysql> select dept_id,count(name) as total from employees group by dept_id having total < (select count(name) from employees where dept_id=(select dept_id from departments where dept_name='开发部'));
+---------+-------+
| dept_id | total |
+---------+-------+
|    NULL |     2 |
|       1 |     8 |
|       2 |     5 |
|       3 |     6 |
|       5 |    12 |
|       6 |     9 |
|       7 |    35 |
|       8 |     3 |
|       9 |     1 |
|      11 |     1 |
+---------+-------+
10 rows in set (0.00 sec)

from 命令之后

把查询结果当作表使用

select 命令之后

把查询结果当做表头使用

select 表头名,(select查询命令) from 库.表 where 条件;