Mysql---查询进阶（二）

内连接之非等值连接： 使用非相等做判断做连接条件

环境准备：创建工资等级表

mysql>use tarena;
mysql> create table wage_grade(
id int  primary key  auto_increment,  grade char(1), low int ,  high int  );   mysql>insert into  wage_grade(grade,low,high)  values('A', 5000, 8000),('B', 8001, 10000),('C', 10001, 15000),('D', 15001, 20000),
('E', 20001, 1000000);mysql> select  * from  wage_grade;
+----+-------+-------+---------+
| id | grade | low   | high    |
+----+-------+-------+---------+
|  1 | A     |  5000 |    8000 |
|  2 | B     |  8001 |   10000 |
|  3 | C     | 10001 |   15000 |
|  4 | D     | 15001 |   20000 |
|  5 | E     | 20001 | 1000000 |
+----+-------+-------+---------+
5 rows in set (0.00 sec)

查询2018年12月员工基本工资级别

select employee_id, date, basic, grade from salary as s   inner join wage_grade as g on s.basic between g.low and g.high where year(date)=2018 and month(date)=12;

查询2018年12月员工各基本工资级别的人数

select grade, count(employee_id)  as  numbersfrom salary as s inner join wage_grade as g  on s.basic between g.low and g.highwhere year(date)=2018 and month(date)=12 group by grade;

查询2018年12月员工基本工资级别，员工需要显示姓名

select name, date, basic, grade from employees as e  inner join salary as  s  on e.employee_id=s.employee_id inner join wage_grade as g  on s.basic between g.low and g.high  where year(date)=2018 and month(date)=12;

给表头定义别名

mysql> select name as 姓名, date as 发工资日期, basic as 基本工资, grade as 工资等级   from employees as e  inner join salary as  s  on e.employee_id=s.employee_id   inner join wage_grade as g  on s.basic between g.low and g.high    where year(date)=2018 and month(date)=12 ；

内连接之自连接：

自己连接自己把1张当2张表使用，实现的方法

就是查询表的时候给表定义别名来实现。

查看哪些员工的生日月份与入职月份相同

select e.name, e.hire_date, em.birth_date
from employees as e  inner join employees as em
on month(e.hire_date)=month(em.birth_date) and  e.employee_id=em.employee_id;

或 where 条件

mysql> select  name ,hire_date , birth_date from  employees -> where month(hire_date) = month(birth_date);

外连接分类如下:

左外连接 LEFT JOIN

左边表的记录全都显示出来右边的表只显示与条件匹配记录，右边表比左边表少的记录使用NULL匹配

右外连接 RIGHT JOIN

右边表的记录全都显示出来左边的表只显示与条件匹配记录，左表比右边表少的记录使用NULL 匹配

全外连接(mysql不支持，可以使用UNION实现相同的效果) ：合并查询结果

外连接的应用场景：

1）比较2个表里记录的不同

2）者哪些数据当前表有而另一张表没有。

环境准备：

向departments表里添加3个部门行政部小卖部公关部

mysql> insert into departments(dept_name) values("小卖部"),("行政部"),("公关部");
mysql> select name,dept_id from employees where name in ("bob","tom","lily");

+------+---------+
| name | dept_id |
+------+---------+
| bob  |    NULL |
| tom  |    NULL |
| lily |    NULL |
+------+---------+
3 rows in set (0.00 sec)
mysql>

向employees表中添加3个员工只给name字段赋值

mysql> insert  into  employees(name)values("bob"),("tom"),("lily");
mysql> select  * from tarena.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)
mysql>

查看哪些员工没有部门

mysql> select e.name,d.dept_name from departments as d  right  join employees as e on d.dept_id=e.dept_id;
...
...
| 刘倩      | 法务部    |
| 杨金凤    | 法务部    |
| bob       | NULL      |
| tom       | NULL      |
| lily      | NULL      |
+-----------+-----------+
136 rows in set (0.00 sec)mysql> select e.name,d.dept_name from departments as d  right  join employees as e on d.dept_id=e.dept_id
where d.dept_name is null ;
+------+-----------+
| name | dept_name |
+------+-----------+
| bob  | NULL      |
| tom  | NULL      |
| lily | NULL      |
+------+-----------+
3 rows in set (0.00 sec)

查看哪些部门没有员工

mysql> select d.dept_name,e.name from departments as d  left  join employees as e on d.dept_id=e.dept_id;
...
...
| 法务部    | 王荣      |
| 法务部    | 刘倩      |
| 法务部    | 杨金凤    |
| 小卖部    | NULL      |
| 行政部    | NULL      |
| 公关部    | NULL      |
+-----------+-----------+
136 rows in set (0.00 sec)mysql> select d.dept_name,e.name from departments as d  left  join employees as e on d.dept_id=e.dept_id where e.name is null;
+-----------+------+
| dept_name | name |
+-----------+------+
| 小卖部    | NULL |
| 行政部    | NULL |
| 公关部    | NULL |
+-----------+------+
3 rows in set (0.01 sec)mysql>

联合查询

也称联合查询，用来合并查询结果

可以合并同一张的表的查询记录（不同表的查询记录也可合并）

要求查询时，多个select语句的检索到的字段数量必须一致

每一条记录的各字段类型和顺序最好是一致的

UNION关键字默认去重，可以使用UNION ALL包含重复项

语法格式 1  (SELECT语句 ) UNION (SELECT语句);
语法格式 2  (SELECT语句 ) UNION  ALL (SELECT语句);

语法格式演示：

mysql> select  "abc";
+-----+
| abc |
+-----+
| abc |
+-----+
1 row in set (0.00 sec)mysql> select  "bcd";
+-----+
| bcd |
+-----+
| bcd |
+-----+
1 row in set (0.00 sec)mysql> (select  "bcd") union (select "xyz");
+-----+
| bcd |
+-----+
| bcd |
| xyz |
+-----+
2 rows in set (0.00 sec)mysql>
mysql> (select  "xyz") union (select "xyz");
+-----+
| xyz |
+-----+
| xyz |
+-----+
1 row in set (0.00 sec)mysql> (select  "xyz") union all (select "xyz");
+-----+
| xyz |
+-----+
| xyz |
| xyz |
+-----+
2 rows in set (0.00 sec)mysql>

查询 1972年或 2000年后出生的员工

使用where 条件查询

select name, birth_date from employees
where year(birth_date)=1972  or  year(birth_date)>2000;

或使用联合查询

( select name, birth_date from employees where year(birth_date)=1972)
union
(select name, birth_date from employees where year(birth_date)>2000 );

一起输出user表中uid号最小和uid号最大的用户名和uid号

select min(uid) from user;
select max(uid) from user;(select name, uid from  user where uid = (select min(uid) from user))
union
(select name, uid from  user where uid = (select max(uid) from user));

嵌套查询

查询运维部所有员工信息（部门名称是运维部的员工信息）

#先把 运维部的id 找到select dept_id from departments where dept_name="运维部";#员工表里没有部门名称 但有部门编号 （和部门表的编号是一致的）select  *  from  employees   where  dept_id = (select dept_id from departments where dept_name="运维部");

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
select basic from salary  where year(date)=2018 and
month(date)=12 and employee_id=100;  #查看比100号员工工资高的
select  *  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);

查询部门员工人数比开发部人数少的部门

#统计每个部门的人数 select   dept_id , count(name) from employees group by   dept_id  ；#统计开发部 员工人数mysql> select count(*) from employees where dept_id = (select dept_id from departments where dept_name="开发部");#输出人数比开发部少的 部门 及 人数
select  dept_id ,  count(name)  as 部门人数   from employees group by dept_id
having count(name)<(
select count(name) from employees  where dept_id=(
select dept_id from departments where dept_name='开发部')
);

查询每个部门的人数

#显示部门表中的所有列表select d.*  from departments as d;   #查询每个部门的人数
select  d.* , ( select count(name) from employees as e  where d.dept_id=e.dept_id) as 总人数
from departments as d;

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idselect dept_id from departments  where dept_name in ('人事部', '财务部');#查询人事部和财务部员工信息
select dept_id , name  from employees
where dept_id in (
select dept_id from departments  where dept_name in ('人事部', '财务部') );

查询人事部2018年12月所有员工工资

select   *   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') );

查找2018年12月基本工资和奖金都是最高的工资信息

select * from salary
where  year(date)=2018 and month(date)=12  and
basic=(select max(basic) from salary where year(date)=2018 and month(date)=12 )
and
bonus=(select max(bonus) from salary where year(date)=2018 and month(date)=12);

查询3号部门及其部门内员工的编号、名字和 email

select dept_id, dept_name, employee_id, name, email  from (
select  d.dept_name, e.*  from departments as d  inner join employees as e
on d.dept_id=e.dept_id ) as tmp_table  where dept_id=3;

多表更新与删除

一起修改或删除多张表里的数据

环境准备：

mysql> use  tarena;
mysql> create  table  t3  select  name , uid  from  tarena.user  limit  2;
mysql> create  table  t4  select  uid,homedir,shell  from  tarena.user  limit  4;
mysql> select  * from  t3 ;
mysql> select  * from t4;

uid 字段是 t3 和 t4 表的关联字段

select * from t3 inner join t4 on  t3.uid = t4.uid ;#多表修改update   t3 inner join t4 on  t3.uid = t4.uid  set  t3.uid=101 , t4.uid=102   where t3.uid=0 ;#查看修改select  * from t3 ;   原先UID=0 没了  看到是  UID  = 101select  * from t4;   原先UID=0 没了  看到   UID  = 102

#删除时使用的多表连接命令

 mysql> select  * from  t3 inner join t4 on  t3.uid = t4.uid;
+------+------+------+---------+---------------+
| name | uid  | uid  | homedir | shell         |
+------+------+------+---------+---------------+
| bin  |    1 |    1 | /bin    | /sbin/nologin |
+------+------+------+---------+---------------+
1 row in set (0.00 sec)

#多表删除

 mysql> delete t3,t4  from t3 inner join t4 on  t3.uid = t4.uid;
Query OK, 2 rows affected (0.06 sec)mysql> select  * from t3;
+------+------+
| name | uid  |
+------+------+
| root |  101 |
+------+------+
1 row in set (0.00 sec)mysql> select  * from t4;
+------+----------+---------------+
| uid  | homedir  | shell         |
+------+----------+---------------+
|  102 | /root    | /bin/bash     |
|    2 | /sbin    | /sbin/nologin |
|    3 | /var/adm | /sbin/nologin |
+------+----------+---------------+
3 rows in set (0.00 sec)mysql>

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