MySQL——分组函数、distinct、分组查询、连接查询、子查询

博客目录

1、分组函数

2、distinct

3、分组查询(group by、having)

4、一个完整的DQL语句的总结

5、连接查询(跨表查询)（关联多个表查询）

6、子查询

---

一、分组函数

1、 会自动忽略空值

2、 分组函数不能直接使用在where关键字后面

• 取平均值：avg

  mysql> select avg(sal) from emp;
  +-------------+
  | avg(sal)    |
  +-------------+
  | 2073.214286 |
  +-------------+
  1 row in set (0.60 sec)
  

• 最大值：max

• 最小值：min

• 取记录数：count

   mysql> select count(sal) from emp;
  +------------+
  | count(sal) |
  +------------+
  |         14 |
  +------------+
  1 row in set (0.04 sec)
  

• 求和：sum

---

二、distinct(去除重复记录)(显示)

使用distinct需要注意

1、该关键字前面不能出现字段

2、该关键字只能出现在所有字段的最前面

mysql> select distinct job from emp;

+-----------+
| job       |
+-----------+
| CLERK     |
| SALESMAN  |
| MANAGER   |
| ANALYST   |
| PRESIDENT |
+-----------+
5 rows in set (0.09 sec)

例

统计公司一共有几个工作岗位

mysql> select count(distinct job) from emp;
+---------------------+
| count(distinct job) |
+---------------------+
|                   5 |
+---------------------+
1 row in set (0.00 sec)

---

三、分组查询(group by、having)

1、group by

通过哪个或者哪些字段进行分组

重点

若一条DQL语句中有group by子句，那么select关键字后面只能跟参与分组的字段和分组函数(比如ename这些是不可以的，不是分组字段)

例

1、找出每个工作岗位的最高薪水

分析：先按照工作岗位分组，再使用max求每组最高记录

    mysql> select job,max(sal) from emp group by job;
    +-----------+----------+
    | job       | max(sal) |
    +-----------+----------+
    | ANALYST   |  3000.00 |
    | CLERK     |  1300.00 |
    | MANAGER   |  2975.00 |
    | PRESIDENT |  5000.00 |
    | SALESMAN  |  1600.00 |
    +-----------+----------+
    5 rows in set (0.00 sec)

2、计算每个部门的平均薪水

mysql> select
    -> deptno,avg(sal) as avgsal
    -> from
    -> emp
    -> group by
    -> deptno;
+--------+-------------+
| deptno | avgsal      |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
|     30 | 1566.666667 |
+--------+-------------+
3 rows in set (0.00 sec)

3、计算不同工作岗位的最高薪水

mysql> select
    -> deptno,job,avg(sal) as avgsal
    -> from
    -> emp
    -> group by
    -> deptno,job;
+--------+-----------+-------------+
| deptno | job       | avgsal      |
+--------+-----------+-------------+
|     10 | CLERK     | 1300.000000 |
|     10 | MANAGER   | 2450.000000 |
|     10 | PRESIDENT | 5000.000000 |
|     20 | ANALYST   | 3000.000000 |
|     20 | CLERK     |  950.000000 |
|     20 | MANAGER   | 2975.000000 |
|     30 | CLERK     |  950.000000 |
|     30 | MANAGER   | 2850.000000 |
|     30 | SALESMAN  | 1400.000000 |
+--------+-----------+-------------+
9 rows in set (0.04 sec)

---

2、having

1. having和where功能都是为了完成数据的过滤
2. where和having后面都是添加条件
3. where和group by之前完成过滤
4. having在group by之后完成过滤

例

找出每个工作岗位的最高薪水，除manager之外

mysql> select
    -> job,avg(sal)
    -> from
    -> emp
    -> group by
    -> job
    -> having
    -> avg(sal)>1500;
+-----------+-------------+
| job       | avg(sal)    |
+-----------+-------------+
| ANALYST   | 3000.000000 |
| MANAGER   | 2758.333333 |
| PRESIDENT | 5000.000000 |
+-----------+-------------+
3 rows in set (0.09 sec)

注意上面为什么不能用where，因为where关键字后面不能用分组函数，分组函数必须在分组完成后执行，而分组要group by。而group by在where后执行

原则

一般情况尽量在where中过滤，实在无法过滤的数据，通常是需要先分组之后再过滤，此时可以选择having，这牵扯到效率问题

---

四、一个完整的DQL语句的总结

select   
    ...
from    从某张表中检索数据
    ...
where   经过某条件进行过滤
    ...
group by    分组
    ...
having      分组之后不满意再过滤
    ...
order by    排序输出
    ...

以上关键字顺序不能改变，严格遵守

以下是执行顺序

1、from
2、where
3、group by
4、having
5、select
6、order by

---

五、连接查询(跨表查询)(关联多个表查询)

1、连接查询分类(年代)

• SQL92
• SQL99

2、连接查询根据连接方式分类

• 内连接(两张表在进行表连接的时候，连接记录百分百匹配记录可查，内连接只能是两表匹配的)
  
  • 等值连接
  • 非等值连接
  • 自连接
• 外链接(两表进行外连接的时候，除了可以百分百记录查询，另外需要将其中任意一张表数据无条件的全部展示出来，就要使用外连接)
  
  • 左外连接(左连接)：将左边数据全部显示
  • 右外连接(右连接)：将右边数据全部显示
• 全连接

外连接的查询结果都是一直大于等于内连接的查询结果

3、当多张表进行连接查询，若没有任何条件进行限制，会发生什么现象

会发生总记录是两张表记录乘积(此现象是笛卡尔积现象，为了避免该现象发生，必须在表连接时候加上限制)

例

1、内连接（等值连接）—————查询每个员工所在的部门名称，要求最终显示员工名和对应的部门名(两张表：员工表(部门号)、部门表(部门号))

tip：在连接查询的时候虽然使用了限制条件，但是匹配的次数没有减少，还是与无限制一样，只是这次的结果都是有效记录

• SQL92语法：（属于内连接中的等值连接）（缺点：表连接与过滤同时了，结构不清晰）

  mysql> select
      -> e.ename,d.dname
      -> from
      -> emp e,dept d
      -> where
      -> e.deptno=d.deptno;
  +--------+------------+
  | ename  | dname      |
  +--------+------------+
  | CLARK  | ACCOUNTING |
  | KING   | ACCOUNTING |
  | MILLER | ACCOUNTING |
  | SMITH  | RESEARCH   |
  | JONES  | RESEARCH   |
  | SCOTT  | RESEARCH   |
  | ADAMS  | RESEARCH   |
  | FORD   | RESEARCH   |
  | ALLEN  | SALES      |
  | WARD   | SALES      |
  | MARTIN | SALES      |
  | BLAKE  | SALES      |
  | TURNER | SALES      |
  | JAMES  | SALES      |
  +--------+------------+
  14 rows in set (0.15 sec)
  

• SQL99语法(inner可以省略)（好处：表连接与过滤分离，在最后加where即可过滤，结构清晰）

  mysql> select
      -> e.ename,d.dname
      -> from
      -> emp e
      -> inner join
      -> dept d
      -> on
      -> e.deptno=d.deptno;
  +--------+------------+
  | ename  | dname      |
  +--------+------------+
  | CLARK  | ACCOUNTING |
  | KING   | ACCOUNTING |
  | MILLER | ACCOUNTING |
  | SMITH  | RESEARCH   |
  | JONES  | RESEARCH   |
  | SCOTT  | RESEARCH   |
  | ADAMS  | RESEARCH   |
  | FORD   | RESEARCH   |
  | ALLEN  | SALES      |
  | WARD   | SALES      |
  | MARTIN | SALES      |
  | BLAKE  | SALES      |
  | TURNER | SALES      |
  | JAMES  | SALES      |
  +--------+------------+
  14 rows in set (0.00 sec)
  

2、内连接（非等值连接，SQL99）————找出每一个员工对应的工资等级，要求显示员工名，工资，工资等级（两张表：员工表，工资等级表(salgrade)）

mysql> select
    -> e.ename,e.sal,s.grade
    -> from
    -> emp e
    -> join
    -> salgrade s
    -> on
    -> e.sal
    -> between
    -> s.losal
    -> and
    -> s.hisal;
+--------+---------+-------+
| ename  | sal     | grade |
+--------+---------+-------+
| SMITH  |  800.00 |     1 |
| ALLEN  | 1600.00 |     3 |
| WARD   | 1250.00 |     2 |
| JONES  | 2975.00 |     4 |
| MARTIN | 1250.00 |     2 |
| BLAKE  | 2850.00 |     4 |
| CLARK  | 2450.00 |     4 |
| SCOTT  | 3000.00 |     4 |
| KING   | 5000.00 |     5 |
| TURNER | 1500.00 |     3 |
| ADAMS  | 1100.00 |     1 |
| JAMES  |  950.00 |     1 |
| FORD   | 3000.00 |     4 |
| MILLER | 1300.00 |     2 |
+--------+---------+-------+
14 rows in set (0.03 sec)

3、内连接(自连接,SQL99)——找出每个员工的上级领导，要求显示员工名以及对应的领导名(一张表看成两张表)

以下是员工表，表中也有员工序号和对应上级的序号，即可看出两张表

mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO
+-------+--------+-----------+------+------------+---------+---------+--------
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10
+-------+--------+-----------+------+------------+---------+---------+--------
14 rows in set (0.00 sec)



mysql> select
    -> a.ename as empname,b.ename as leadername
    -> from
    -> emp a
    -> join
    -> emp b
    -> on
    -> a.mgr=b.empno;
+---------+------------+
| empname | leadername |
+---------+------------+
| SMITH   | FORD       |
| ALLEN   | BLAKE      |
| WARD    | BLAKE      |
| JONES   | KING       |
| MARTIN  | BLAKE      |
| BLAKE   | KING       |
| CLARK   | KING       |
| SCOTT   | JONES      |
| TURNER  | BLAKE      |
| ADAMS   | SCOTT      |
| JAMES   | BLAKE      |
| FORD    | JONES      |
| MILLER  | CLARK      |
+---------+------------+
13 rows in set (0.00 sec)

SQL92语法就是将join改成’,’，on改成where

4、外连接(右连接)——找出每个员工对应的部门名称，要求部门名称全部显示（部分多出来的记录将用null与其匹配）(左连接的话将dept和emp换个位置就好)

mysql> select
    -> e.ename,d.dname
    -> from
    -> emp e
    -> right outer join
    -> dept d
    -> on
    -> e.deptno=d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| CLARK  | ACCOUNTING |
| KING   | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH  | RESEARCH   |
| JONES  | RESEARCH   |
| SCOTT  | RESEARCH   |
| ADAMS  | RESEARCH   |
| FORD   | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| TURNER | SALES      |
| JAMES  | SALES      |
| NULL   | OPERATIONS |
+--------+------------+
15 rows in set (0.05 sec)

5、找出每个员工对应的领导名，要求显示所有的员工

mysql> select a.ename empname,b.ename leadername from emp a join emp b on a.mgr=
b.empno;
+---------+------------+
| empname | leadername |
+---------+------------+
| SMITH   | FORD       |
| ALLEN   | BLAKE      |
| WARD    | BLAKE      |
| JONES   | KING       |
| MARTIN  | BLAKE      |
| BLAKE   | KING       |
| CLARK   | KING       |
| SCOTT   | JONES      |
| TURNER  | BLAKE      |
| ADAMS   | SCOTT      |
| JAMES   | BLAKE      |
| FORD    | JONES      |
| MILLER  | CLARK      |
+---------+------------+
13 rows in set (0.00 sec)

（多张表）6、找出每个员工对应的部门名称，以及该员工对应的部门名称，工资等级。要求显示员工名、部门名、工资等级

多张表进行表连接的语法格式(a,b,c表)
原理：a与b连接后，再a与c连接

select
    ...
from 
    a
join
    b
on
    条件
join
    c
on
    条件；

mysql> select e.ename,d.dname,s.grade from emp e join dept d on e.deptno=d.deptn
o join salgrade s on e.sal between s.losal and s.hisal;
+--------+------------+-------+
| ename  | dname      | grade |
+--------+------------+-------+
| SMITH  | RESEARCH   |     1 |
| ALLEN  | SALES      |     3 |
| WARD   | SALES      |     2 |
| JONES  | RESEARCH   |     4 |
| MARTIN | SALES      |     2 |
| BLAKE  | SALES      |     4 |
| CLARK  | ACCOUNTING |     4 |
| SCOTT  | RESEARCH   |     4 |
| KING   | ACCOUNTING |     5 |
| TURNER | SALES      |     3 |
| ADAMS  | RESEARCH   |     1 |
| JAMES  | SALES      |     1 |
| FORD   | RESEARCH   |     4 |
| MILLER | ACCOUNTING |     2 |
+--------+------------+-------+
14 rows in set (0.05 sec)

TIP

为什么inner、outer可以省略

因为区分内连接和外连接依靠的不是这些关键字，而是看SQL语句中是否存在left/right，若存在，标识一定是个外连接，其他都是内连接

inner、outer加上有什么好处

增强可读性

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六、子查询

子查询就是嵌套的select语句(select语句嵌套select语句)

1、子查询出现的位置

• select…(select)
• from…(select)
• where…(select)

例

where后使用子查询

找出薪水比公司平均薪水高的员工，要求显示员工名和薪水

• 第一步找出公司的平均水平

  select avg(sal) from emp;
  

• 第二步找出薪水大于平均薪水的员工信息

  mysql> select ename,sal from emp where sal>(select avg(sal) from emp);
  +-------+---------+
  | ename | sal     |
  +-------+---------+
  | JONES | 2975.00 |
  | BLAKE | 2850.00 |
  | CLARK | 2450.00 |
  | SCOTT | 3000.00 |
  | KING  | 5000.00 |
  | FORD  | 3000.00 |
  +-------+---------+
  6 rows in set (0.00 sec)
  

from后面使用子查询（可将查询结果当做临时表）

找出每个部门的平均薪水，并找出平均薪水的薪水等级

• 第一步找出每个部门的平均薪水

  mysql> select avg(sal) as avgsal from emp group by deptno;
  

• 第二步将上面的查询结果当做临时表t，t表和salgrade s表进行表连接，条件：t.avgsal between s.losal and s.hisal

  mysql> select
      -> t.deptno,t.avgsal,s.grade
      -> from
      -> (select deptno,avg(sal) as avgsal from emp group by deptno) t
      -> join
      -> salgrade s
      -> on
      -> t.avgsal
      -> between
      -> s.losal
      -> and
      -> s.hisal;
  +--------+-------------+-------+
  | deptno | avgsal      | grade |
  +--------+-------------+-------+
  |     30 | 1566.666667 |     3 |
  |     10 | 2916.666667 |     4 |
  |     20 | 2175.000000 |     4 |
  +--------+-------------+-------+
  3 rows in set (0.12 sec)
  

select后使用子查询(了解)

显示员工名与其对应的部门名

mysql> select e.ename,(select d.dname from dept d where e.deptno=d.deptno) as dn
ame from emp e;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| JONES  | RESEARCH   |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| CLARK  | ACCOUNTING |
| SCOTT  | RESEARCH   |
| KING   | ACCOUNTING |
| TURNER | SALES      |
| ADAMS  | RESEARCH   |
| JAMES  | SALES      |
| FORD   | RESEARCH   |
| MILLER | ACCOUNTING |
+--------+------------+
14 rows in set (0.00 sec)