MySQL8 常用命令之DQL（多表联查）

DQL：（Data Query Language）数据查询语言

单表查询

一、去除重复记录

使用distinct关键字

select distinct 字段名 from表名：

注：

distinct 只对查询结果进行去重，原表数据不会被修改。
distinct 后面可以跟多个字段，表示多个字段联合去重。

mysql> select distinct job,deptno from emp order by deptno;
+-----------+--------+
| job       | deptno |
+-----------+--------+
| CLERK     |     10 |
| MANAGER   |     10 |
| PRESIDENT |     10 |
| ANALYST   |     20 |
| CLERK     |     20 |
| MANAGER   |     20 |
| CLERK     |     30 |
| MANAGER   |     30 |
| SALESMAN  |     30 |
+-----------+--------+
9 rows in set (0.01 sec)

二、连接查询

1. 笛卡尔积现象

当两张表进行连接查询时，如果没有任何条件的限制会发生笛卡尔积现象。如emp表有14条记录，dept表有4条记录，不加任何条件直接在两表查询最终会显示56条数据。

mysql> select ename,dname from emp,dept;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | OPERATIONS |
| SMITH  | SALES      |
| SMITH  | RESEARCH   |
| SMITH  | ACCOUNTING |
| ALLEN  | OPERATIONS |
......
| MILLER | RESEARCH   |
| MILLER | ACCOUNTING |
+--------+------------+
56 rows in set (0.00 sec)

可以通过添加条件避免笛卡尔积现象的发生。

mysql> select e.ename,d.dname from emp e,dept d where e.deptno=d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
......
| JAMES  | SALES      |
| FORD   | RESEARCH   |
| MILLER | ACCOUNTING |
+--------+------------+
14 rows in set (0.01 sec)

2. 内连接

select ... from A inner join B on A和B的连接条件 where 筛选条件;

说明：

inner可以省略，带着inner可读性更好
内连接的特点：AB两表之间没有主次关系，将完全能够满足连接条件的数据查询出来。

2.1 等值连接
on 后面的连接条件是等量关系。

案例：查询员工的部门名称。

mysql> select e.ename,d.dname from emp e inner join dept d on e.deptno=d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
......
| JAMES  | SALES      |
| FORD   | RESEARCH   |
| MILLER | ACCOUNTING |
+--------+------------+
14 rows in set (0.01 sec)

2.2 非等值连接
连接条件不是等量关系。

案例：查询每个员工的薪资等级。

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 |
......
| FORD   | 3000.00 |     4 |
| MILLER | 1300.00 |     2 |
+--------+---------+-------+
14 rows in set (0.01 sec)

2.3 自连接
将一张表看做是两张表。

案例：查询员工的上级领导并显示。

mysql> select e.ename,m.ename from 
	 > (select empno,ename from emp) as m 
	 > join emp e on e.mgr=m.empno;
+--------+-------+
| ename  | ename |
+--------+-------+
| 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)

3. 外连接

两张表具有主次关系。语句中的outer可以省略。
3.1 左外连接

select ... from A left outer join B on 连接条件;

说明：将join关键字左边的表看做是主表，左表中的数据将被全部查询出来，右表中不满足条件的数据使用NULL填充。

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

3.2 右外连接

select ... from A right outer join B on 连接条件;

说明：将join关键字右边的表看做是主表，右表中的数据将被全部查询出来，左表中不满足条件的数据使用NULL填充。

mysql> select e.ename,d.dname from emp e 
	 > right join dept d on e.deptno=d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| MILLER | ACCOUNTING |
| KING   | ACCOUNTING |
| CLARK  | ACCOUNTING |
| FORD   | RESEARCH   |
| ADAMS  | RESEARCH   |
| SCOTT  | RESEARCH   |
| JONES  | RESEARCH   |
| SMITH  | RESEARCH   |
| JAMES  | SALES      |
| TURNER | SALES      |
| BLAKE  | SALES      |
| MARTIN | SALES      |
| WARD   | SALES      |
| ALLEN  | SALES      |
| NULL   | OPERATIONS |
+--------+------------+
15 rows in set (0.01 sec)

案例：查询所有员工（包括KING）的上级领导并显示员工名和领导名。

mysql> select e.ename,m.ename from emp e
    -> left join (select empno,ename from emp) m
    -> on e.mgr=m.empno;
+--------+-------+
| ename  | ename |
+--------+-------+
| SMITH  | FORD  |
| ALLEN  | BLAKE |
| WARD   | BLAKE |
| JONES  | KING  |
| MARTIN | BLAKE |
| BLAKE  | KING  |
| CLARK  | KING  |
| SCOTT  | JONES |
| KING   | NULL  |
| TURNER | BLAKE |
| ADAMS  | SCOTT |
| JAMES  | BLAKE |
| FORD   | JONES |
| MILLER | CLARK |
+--------+-------+
14 rows in set (0.00 sec)

4.三表联查，四表联查…

select ... from A join B on AB连接条件 join C on AC连接条件 right join D on AD连接条件...;

案例1：查询每个员工的部门名称以及工资等级，要求显示员工名、部门名、薪资、薪资等级。

mysql> select e.ename,d.dname,e.sal,s.grade from emp e
    -> join dept d on e.deptno=d.deptno
    -> join salgrade s on e.sal between s.losal and s.hisal;
+--------+------------+---------+-------+
| ename  | dname      | sal     | grade |
+--------+------------+---------+-------+
| SMITH  | RESEARCH   |  800.00 |     1 |
| ALLEN  | SALES      | 1600.00 |     3 |
| WARD   | SALES      | 1250.00 |     2 |
| JONES  | RESEARCH   | 2975.00 |     4 |
| MARTIN | SALES      | 1250.00 |     2 |
| BLAKE  | SALES      | 2850.00 |     4 |
| CLARK  | ACCOUNTING | 2450.00 |     4 |
| SCOTT  | RESEARCH   | 3000.00 |     4 |
| KING   | ACCOUNTING | 5000.00 |     5 |
| TURNER | SALES      | 1500.00 |     3 |
| ADAMS  | RESEARCH   | 1100.00 |     1 |
| JAMES  | SALES      |  950.00 |     1 |
| FORD   | RESEARCH   | 3000.00 |     4 |
| MILLER | ACCOUNTING | 1300.00 |     2 |
+--------+------------+---------+-------+
14 rows in set (0.01 sec)

案例2：查询每个员工的领导名，部门名称，工资以及工资等级。

mysql> select e.ename,m.ename mname,d.dname,e.sal,s.grade from emp e
    -> left join (select empno,ename from emp) as m on e.mgr=m.empno
    -> join dept d on e.deptno=d.deptno
    -> join salgrade s on e.sal between s.losal and s.hisal;
+--------+-------+------------+---------+-------+
| ename  | mname | dname      | sal     | grade |
+--------+-------+------------+---------+-------+
| SMITH  | FORD  | RESEARCH   |  800.00 |     1 |
| ALLEN  | BLAKE | SALES      | 1600.00 |     3 |
| WARD   | BLAKE | SALES      | 1250.00 |     2 |
| JONES  | KING  | RESEARCH   | 2975.00 |     4 |
| MARTIN | BLAKE | SALES      | 1250.00 |     2 |
| BLAKE  | KING  | SALES      | 2850.00 |     4 |
| CLARK  | KING  | ACCOUNTING | 2450.00 |     4 |
| SCOTT  | JONES | RESEARCH   | 3000.00 |     4 |
| KING   | NULL  | ACCOUNTING | 5000.00 |     5 |
| TURNER | BLAKE | SALES      | 1500.00 |     3 |
| ADAMS  | SCOTT | RESEARCH   | 1100.00 |     1 |
| JAMES  | BLAKE | SALES      |  950.00 |     1 |
| FORD   | JONES | RESEARCH   | 3000.00 |     4 |
| MILLER | CLARK | ACCOUNTING | 1300.00 |     2 |
+--------+-------+------------+---------+-------+
14 rows in set (0.00 sec)

三、子查询

select语句中嵌套select语句，称为子查询。

1. where中的子查询

上一篇文章中提到where子句中不能直接使用分组函数，可以使用子查询解决这个问题。

案例：查询比最低工资高的员工姓名和工资。

mysql> select ename,sal from emp
    -> where sal>(select min(sal) from emp);
+--------+---------+
| ename  | sal     |
+--------+---------+
| ALLEN  | 1600.00 |
| WARD   | 1250.00 |
| JONES  | 2975.00 |
| MARTIN | 1250.00 |
| BLAKE  | 2850.00 |
| CLARK  | 2450.00 |
| SCOTT  | 3000.00 |
| KING   | 5000.00 |
| TURNER | 1500.00 |
| ADAMS  | 1100.00 |
| JAMES  |  950.00 |
| FORD   | 3000.00 |
| MILLER | 1300.00 |
+--------+---------+
13 rows in set (0.01 sec)

2. from后的子查询

注：from后面的子查询，可以将子查询结果当作一张临时表。

案例：查询每个岗位的平均工资的薪资等级。

mysql> select a.job,a.avgsal,s.grade from
    -> (select job,round(avg(sal),2) avgsal from emp group by job) a
    -> join salgrade s on a.avgsal between s.losal and s.hisal;
+-----------+---------+-------+
| job       | avgsal  | grade |
+-----------+---------+-------+
| CLERK     |  1037.5 |     1 |
| SALESMAN  |    1400 |     2 |
| MANAGER   | 2758.33 |     4 |
| ANALYST   |    3000 |     4 |
| PRESIDENT |    5000 |     5 |
+-----------+---------+-------+
5 rows in set (0.00 sec)

3. select后的子查询（了解）

案例：查询员工的所属部门。

mysql> select e.ename,(select d.dname from dept d where e.deptno=d.deptno) dname 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)

注：对于select后的子查询来说，一次返回一条数据，多于一条会报错。

四、union 合并查询结果

有ABC三张表，各有10条记录
A 连接 B 连接 C ----- 匹配次数：10*10*10 = 1000次
------------------------------------------------------------------
A 连接 B ----- 匹配次数： 10*10 = 100次
A 连接 C ----- 匹配次数： 10*10 = 100次
使用union合并：100+100 = 200次

案例：查询工作岗位是MANAGER和SALESMAN的员工。

mysql> select ename,job from emp where job='manager' or job='salesman';
mysql> select ename,job from emp where job in ('manager','salesman');

以上两条语句是利用之前提到的知识写出来的，结果相同。

mysql> select ename,job from emp where job='manager'
    -> union
    -> select ename,job from emp where job='salesman';

注：union在进行结果合并时，要求两个结果集的列数相同。在书写语句时查询字段顺序书写要保持一致。

union的查询效率更高，对于表连接来说，每连接一次新表，匹配的次数满足笛卡尔积，也就是说查询次数会成倍增长，但是union会减少查询次数，同时将查询结果集进行拼接输出。

五、分页查询

1. limit(start,len)

说明：

将查询结果的一部分取出来。
start：起始下标，可以省略，默认从0开始。
len：截取长度。
mysql中limit在order by后面执行

案例：查询工资排名前3-5的员工。

mysql> select ename,sal from emp order by sal desc limit 2,3;
+-------+---------+
| ename | sal     |
+-------+---------+
| FORD  | 3000.00 |
| JONES | 2975.00 |
| BLAKE | 2850.00 |
+-------+---------+
3 rows in set (0.00 sec)

2. 通用分页

每页显示pageSize条记录，第pageNo页
公式：limit (pageNo-1) * pageSize,pageSize

DQL总结

1. 查询语句书写顺序

select ...from ...
where ...
group by ... having ...
order by ... limit...

2. 执行顺序

from
where
group by
having
select
order by
limit