复合查询和内外连接

文章目录

1. 简单查询
2. 多表查询
- 2.1 显示雇员名、雇员工资以及所在部门的名字
- 2.2 显示部门号为10的部门名，员工名和工资
- 2.3 显示各个员工的姓名，工资，及工资级别
3. 自连接
4. 子查询
- 4.1 where后的子查询
- - 4.1.1 单行子查询
  - 4.1.2 多行子查询 (in, all, any)
  - 4.1.3 多列子查询
- 4.2 from后的子查询
- 4.3 合并查询
5. 内外连接
- 5.1 内连接
- 5.2 左外连接
- 5.3 右外连接

1. 简单查询

先做几个简单查询的题目，使用下面的emp表

查询工资高于2000或岗位为MANAGER的雇员，同时还要满足他们的姓名首字母为大写的J

mysql> select * from emp where (sal > 2000 or job = "MANAGER") and ename like "J%";
+--------+-------+---------+------+---------------------+---------+------+--------+
| empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+---------+------+---------------------+---------+------+--------+
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
+--------+-------+---------+------+---------------------+---------+------+--------+
-- 也可以写成这样
select * from emp where (sal > 2000 or job = "MANAGER") and left(ename,1)='J';

按照部门号升序而雇员的工资降序排序

mysql> select ename, deptno, sal from emp order by deptno asc, sal desc;
+--------+--------+---------+
| ename  | deptno | sal     |
+--------+--------+---------+
| KING   |     10 | 5000.00 |
| CLARK  |     10 | 2450.00 |
| MILLER |     10 | 1300.00 |
| SCOTT  |     20 | 3000.00 |
| FORD   |     20 | 3000.00 |
| JONES  |     20 | 2975.00 |
| ADAMS  |     20 | 1100.00 |
| SMITH  |     20 |  800.00 |
| BLAKE  |     30 | 2850.00 |
| ALLEN  |     30 | 1600.00 |
| TURNER |     30 | 1500.00 |
| WARD   |     30 | 1250.00 |
| MARTIN |     30 | 1250.00 |
| JAMES  |     30 |  950.00 |
+--------+--------+---------+
14 rows in set (0.00 sec)

使用年薪进行降序排序

-- 年薪计算用sal*12+comm，由于有的列comm为null，所以这里要检测一下
mysql> select ename, sal+ifnull(comm,0) total from emp order by total desc;
+--------+---------+
| ename  | total   |
+--------+---------+
| KING   | 5000.00 |
| SCOTT  | 3000.00 |
| FORD   | 3000.00 |
| JONES  | 2975.00 |
| BLAKE  | 2850.00 |
| MARTIN | 2650.00 |
| CLARK  | 2450.00 |
| ALLEN  | 1900.00 |
| WARD   | 1750.00 |
| TURNER | 1500.00 |
| MILLER | 1300.00 |
| ADAMS  | 1100.00 |
| JAMES  |  950.00 |
| SMITH  |  800.00 |
+--------+---------+
14 rows in set (0.00 sec)

显示工资最高的员工的名字和工作岗位

mysql> select max(sal) from emp;
+----------+
| max(sal) |
+----------+
|  5000.00 |
+----------+
1 row in set (0.00 sec)

mysql> select ename, sal, deptno from emp where sal = 5000;
+-------+---------+--------+
| ename | sal     | deptno |
+-------+---------+--------+
| KING  | 5000.00 |     10 |
+-------+---------+--------+
1 row in set (0.00 sec)

-- 可以嵌套查询
mysql> select ename, sal, deptno from emp where sal = (select max(sal) from emp);
+-------+---------+--------+
| ename | sal     | deptno |
+-------+---------+--------+
| KING  | 5000.00 |     10 |
+-------+---------+--------+
1 row in set (0.00 sec)

显示工资高于平均工资的员工信息

mysql> select empno, ename, job, sal from emp where sal > (select avg(sal) from emp);
+--------+-------+-----------+---------+
| empno  | ename | job       | sal     |
+--------+-------+-----------+---------+
| 007566 | JONES | MANAGER   | 2975.00 |
| 007698 | BLAKE | MANAGER   | 2850.00 |
| 007782 | CLARK | MANAGER   | 2450.00 |
| 007788 | SCOTT | ANALYST   | 3000.00 |
| 007839 | KING  | PRESIDENT | 5000.00 |
| 007902 | FORD  | ANALYST   | 3000.00 |
+--------+-------+-----------+---------+
6 rows in set (0.00 se

显示每个部门的平均工资和最高工资

mysql> select deptno, max(sal), avg(sal) from emp group by (deptno);
+--------+----------+-------------+
| deptno | max(sal) | avg(sal)    |
+--------+----------+-------------+
|     10 |  5000.00 | 2916.666667 |
|     20 |  3000.00 | 2175.000000 |
|     30 |  2850.00 | 1566.666667 |
+--------+----------+-------------+
3 rows in set (0.00 sec)

显示平均工资低于2000的部门号和它的平均工资

mysql> select deptno, avg(sal) avg from emp group by (deptno) having avg < 2000;
+--------+-------------+
| deptno | avg         |
+--------+-------------+
|     30 | 1566.666667 |
+--------+-------------+
1 row in set (0.00 sec)

显示每种岗位的雇员总数，平均工资

mysql> select job, avg(sal), count(*) num from emp group by (job);
+-----------+-------------+-----+
| job       | avg(sal)    | num |
+-----------+-------------+-----+
| ANALYST   | 3000.000000 |   2 |
| CLERK     | 1037.500000 |   4 |
| MANAGER   | 2758.333333 |   3 |
| PRESIDENT | 5000.000000 |   1 |
| SALESMAN  | 1400.000000 |   4 |
+-----------+-------------+-----+
5 rows in set (0.00 sec)

2. 多表查询

现在又有多出来的两张表，进行多表查询

2.1 显示雇员名、雇员工资以及所在部门的名字

显然要用到emp表和dept表，下面的操作叫做求两个表的笛卡尔积
从第一张表取出每一条记录，和第二张表的所有记录进行组合。本质是进行穷举

mysql> select * from emp, dept;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno | deptno | dname      | loc      |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 | 800.00  | NULL    |     20 |     10 | ACCOUNTING | NEW YORK |
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 | 800.00  | NULL    |     20 |     20 | RESEARCH   | DALLAS   |
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 | 800.00  | NULL    |     20 |     30 | SALES      | CHICAGO  |
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 | 800.00  | NULL    |     20 |     40 | OPERATIONS | BOSTON   |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00  |     30 |     10 | ACCOUNTING | NEW YORK |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00  |     30 |     20 | RESEARCH   | DALLAS   |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00  |     30 |     30 | SALES      | CHICAGO  |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00  |     30 |     40 | OPERATIONS | BOSTON   |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00  |     30 |     10 | ACCOUNTING | NEW YORK |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00  |     30 |     20 | RESEARCH   | DALLAS   |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00  |     30 |     30 | SALES      | CHICAGO  |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00  |     30 |     40 | OPERATIONS | BOSTON   |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL    |     20 |     10 | ACCOUNTING | NEW YORK |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL    |     20 |     20 | RESEARCH   | DALLAS   |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL    |     20 |     30 | SALES      | CHICAGO  |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL    |     20 |     40 | OPERATIONS | BOSTON   |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     10 | ACCOUNTING | NEW YORK |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     20 | RESEARCH   | DALLAS   |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     40 | OPERATIONS | BOSTON   |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL    |     30 |     10 | ACCOUNTING | NEW YORK |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL    |     30 |     20 | RESEARCH   | DALLAS   |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL    |     30 |     30 | SALES      | CHICAGO  |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL    |     30 |     40 | OPERATIONS | BOSTON   |
......
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
56 rows in set (1.22 sec)

显然有重复显示的，例如对于SMITH来讲，只需要显示dept.deptno=20的那一行，所以需要加限制条件

mysql> select * from emp, dept where emp.deptno = dept.deptno;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno | deptno | dname      | loc      |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 | 800.00  | NULL    |     20 |     20 | RESEARCH   | DALLAS   |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00  |     30 |     30 | SALES      | CHICAGO  |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00  |     30 |     30 | SALES      | CHICAGO  |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL    |     20 |     20 | RESEARCH   | DALLAS   |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL    |     30 |     30 | SALES      | CHICAGO  |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL    |     10 |     10 | ACCOUNTING | NEW YORK |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL    |     20 |     20 | RESEARCH   | DALLAS   |
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL    |     10 |     10 | ACCOUNTING | NEW YORK |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00    |     30 |     30 | SALES      | CHICAGO  |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL    |     20 |     20 | RESEARCH   | DALLAS   |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 | 950.00  | NULL    |     30 |     30 | SALES      | CHICAGO  |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL    |     20 |     20 | RESEARCH   | DALLAS   |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL    |     10 |     10 | ACCOUNTING | NEW YORK |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
14 rows in set (0.18 sec)

按照题目要求进行显示

mysql> select ename, sal, dname from emp, dept where emp.deptno = dept.deptno;
+--------+---------+------------+
| ename  | sal     | dname      |
+--------+---------+------------+
| SMITH  | 800.00  | RESEARCH   |
| ALLEN  | 1600.00 | SALES      |
| WARD   | 1250.00 | SALES      |
| JONES  | 2975.00 | RESEARCH   |
| MARTIN | 1250.00 | SALES      |
| BLAKE  | 2850.00 | SALES      |
| CLARK  | 2450.00 | ACCOUNTING |
| SCOTT  | 3000.00 | RESEARCH   |
| KING   | 5000.00 | ACCOUNTING |
| TURNER | 1500.00 | SALES      |
| ADAMS  | 1100.00 | RESEARCH   |
| JAMES  | 950.00  | SALES      |
| FORD   | 3000.00 | RESEARCH   |
| MILLER | 1300.00 | ACCOUNTING |
+--------+---------+------------+
14 rows in set (0.15 sec)

2.2 显示部门号为10的部门名，员工名和工资

mysql> select dname, ename, sal from emp, dept where emp.deptno = dept.deptno and emp.deptno = 10;
+------------+--------+---------+
| dname      | ename  | sal     |
+------------+--------+---------+
| ACCOUNTING | CLARK  | 2450.00 |
| ACCOUNTING | KING   | 5000.00 |
| ACCOUNTING | MILLER | 1300.00 |
+------------+--------+---------+
3 rows in set (0.16 sec)

2.3 显示各个员工的姓名，工资，及工资级别

这里要用到salgrade表

mysql> select ename, sal, grade, losal, hisal from emp, salgrade hisal where sal between losal and hisal;
+--------+---------+-------+-------+-------+
| ename  | sal     | grade | losal | hisal |
+--------+---------+-------+-------+-------+
| SMITH  | 800.00  |     1 |   700 |  1200 |
| ALLEN  | 1600.00 |     3 |  1401 |  2000 |
| WARD   | 1250.00 |     2 |  1201 |  1400 |
| JONES  | 2975.00 |     4 |  2001 |  3000 |
| MARTIN | 1250.00 |     2 |  1201 |  1400 |
| BLAKE  | 2850.00 |     4 |  2001 |  3000 |
| CLARK  | 2450.00 |     4 |  2001 |  3000 |
| SCOTT  | 3000.00 |     4 |  2001 |  3000 |
| KING   | 5000.00 |     5 |  3001 |  9999 |
| TURNER | 1500.00 |     3 |  1401 |  2000 |
| ADAMS  | 1100.00 |     1 |   700 |  1200 |
| JAMES  | 950.00  |     1 |   700 |  1200 |
| FORD   | 3000.00 |     4 |  2001 |  3000 |
| MILLER | 1300.00 |     2 |  1201 |  1400 |
+--------+---------+-------+-------+-------+
14 rows in set (0.12 sec)

3. 自连接

也可以自己连接自己，用dept表实例

-- 这样是不行的
mysql> select * from dept, dept;
1066 - Not unique table/alias: 'dept'

-- 需要取别名
mysql> select * from dept t1, dept t2;
+--------+------------+----------+--------+------------+----------+
| deptno | dname      | loc      | deptno | dname      | loc      |
+--------+------------+----------+--------+------------+----------+
|     10 | ACCOUNTING | NEW YORK |     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |     10 | ACCOUNTING | NEW YORK |
|     30 | SALES      | CHICAGO  |     10 | ACCOUNTING | NEW YORK |
|     40 | OPERATIONS | BOSTON   |     10 | ACCOUNTING | NEW YORK |
|     10 | ACCOUNTING | NEW YORK |     20 | RESEARCH   | DALLAS   |
|     20 | RESEARCH   | DALLAS   |     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |     20 | RESEARCH   | DALLAS   |
|     40 | OPERATIONS | BOSTON   |     20 | RESEARCH   | DALLAS   |
|     10 | ACCOUNTING | NEW YORK |     30 | SALES      | CHICAGO  |
|     20 | RESEARCH   | DALLAS   |     30 | SALES      | CHICAGO  |
|     30 | SALES      | CHICAGO  |     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |     30 | SALES      | CHICAGO  |
|     10 | ACCOUNTING | NEW YORK |     40 | OPERATIONS | BOSTON   |
|     20 | RESEARCH   | DALLAS   |     40 | OPERATIONS | BOSTON   |
|     30 | SALES      | CHICAGO  |     40 | OPERATIONS | BOSTON   |
|     40 | OPERATIONS | BOSTON   |     40 | OPERATIONS | BOSTON   |
+--------+------------+----------+--------+------------+----------+
16 rows in set (0.15 sec)

显示员工FORD的上级领导的编号和姓名

-- 可以使用子查询
mysql> select empno, ename from emp where empno=(select mgr from emp where ename='FORD');
+--------+-------+
| empno  | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
1 row in set (0.13 sec)

-- 自连接
mysql> select e2.empno, e2.ename from emp e1, emp e2 where e1.mgr=e2.empno and e1.ename='FORD';
+--------+-------+
| empno  | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
1 row in set (0.14 sec)

4. 子查询

子查询（Subquery）也被称为嵌套查询（Nested Query）。它是一个嵌套在另一个 SQL 查询（通常是SELECT语句）中的完整 SQL 查询。

4.1 where后的子查询

4.1.1 单行子查询

子查询返回一行记录

显示SMITH同一部门的员工

-- 返回一行记录
mysql> select deptno from emp where ename = 'SMITH';
+--------+
| deptno |
+--------+
|     20 |
+--------+
1 row in set (0.06 sec)

mysql> select * from emp where deptno = (select deptno from emp where ename = 'SMITH');
+--------+-------+---------+------+---------------------+---------+------+--------+
| empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+---------+------+---------------------+---------+------+--------+
| 007369 | SMITH | CLERK   | 7902 | 1980-12-17 00:00:00 | 800.00  | NULL |     20 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |
| 007876 | ADAMS | CLERK   | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL |     20 |
| 007902 | FORD  | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
+--------+-------+---------+------+---------------------+---------+------+--------+
5 rows in set (0.07 sec)

4.1.2 多行子查询 (in, all, any)

子查询返回多行记录

使用in关键字：查询和10号部门的工作岗位相同的雇员的名字，岗位，工资，部门号，但是不包含10自己的

-- 返回多行记录
mysql> select job from emp where deptno = 10;
+-----------+
| job       |
+-----------+
| MANAGER   |
| PRESIDENT |
| CLERK     |
+-----------+
3 rows in set (0.08 sec)

mysql> select ename, job, sal, deptno from emp where job in (select job from emp where deptno = 10) and deptno != 10;
+-------+---------+---------+--------+
| ename | job     | sal     | deptno |
+-------+---------+---------+--------+
| JONES | MANAGER | 2975.00 |     20 |
| BLAKE | MANAGER | 2850.00 |     30 |
| SMITH | CLERK   | 800.00  |     20 |
| ADAMS | CLERK   | 1100.00 |     20 |
| JAMES | CLERK   | 950.00  |     30 |
+-------+---------+---------+--------+
5 rows in set (0.07 sec)

注：如果想要加上部门名称，可以这样写，就是上面的多表查询

mysql> select ename, job, sal, emp.deptno, dname from emp, dept where emp.deptno=dept.deptno and job in (select job from emp where emp.deptno = 10) and emp.deptno != 10;
+-------+---------+---------+--------+----------+
| ename | job     | sal     | deptno | dname    |
+-------+---------+---------+--------+----------+
| SMITH | CLERK   | 800.00  |     20 | RESEARCH |
| JONES | MANAGER | 2975.00 |     20 | RESEARCH |
| ADAMS | CLERK   | 1100.00 |     20 | RESEARCH |
| BLAKE | MANAGER | 2850.00 |     30 | SALES    |
| JAMES | CLERK   | 950.00  |     30 | SALES    |
+-------+---------+---------+--------+----------+
5 rows in set (0.08 sec)

-- 实际上，from后面也可以跟子查询，因为子查询返回的是一个完整的表, 下面返回结果与上面是一样的
select ename, job, sal, tmp.deptno, dname from (select ename, job, sal, deptno from emp where job in (select job from emp where deptno = 10) and deptno != 10) as tmp, dept where dept.deptno = tmp.deptno;

使用all关键字：显示工资比部门30的所有员工的工资高的员工的姓名、工资和部门号

mysql> select sal from emp where deptno = 30;
+---------+
| sal     |
+---------+
| 1600.00 |
| 1250.00 |
| 1250.00 |
| 2850.00 |
| 1500.00 |
| 950.00  |
+---------+
6 rows in set (0.10 sec)

mysql> select ename, sal, deptno from emp where sal > all(select sal from emp where deptno = 30);
+-------+---------+--------+
| ename | sal     | deptno |
+-------+---------+--------+
| JONES | 2975.00 |     20 |
| SCOTT | 3000.00 |     20 |
| KING  | 5000.00 |     10 |
| FORD  | 3000.00 |     20 |
+-------+---------+--------+
4 rows in set (0.10 sec)

-- 当然也可以这样写
mysql> select ename, sal, deptno from emp where sal > (select max(sal) from emp where deptno = 30);

使用any关键字: 显示工资比部门30的任意员工的工资高的员工的姓名、工资和部门号（包含自己部门的员工）

mysql> select ename, sal, deptno from emp where sal > any(select sal from emp where deptno = 30);
+--------+---------+--------+
| ename  | sal     | deptno |
+--------+---------+--------+
| ALLEN  | 1600.00 |     30 |
| WARD   | 1250.00 |     30 |
| JONES  | 2975.00 |     20 |
| MARTIN | 1250.00 |     30 |
| BLAKE  | 2850.00 |     30 |
| CLARK  | 2450.00 |     10 |
| SCOTT  | 3000.00 |     20 |
| KING   | 5000.00 |     10 |
| TURNER | 1500.00 |     30 |
| ADAMS  | 1100.00 |     20 |
| FORD   | 3000.00 |     20 |
| MILLER | 1300.00 |     10 |
+--------+---------+--------+
12 rows in set (0.11 sec)

4.1.3 多列子查询

子查询返回多列记录

查询和SMITH的部门和岗位完全相同的所有雇员，不含SMITH本人

mysql> select deptno, job from emp where ename = 'SMITH';
+--------+-------+
| deptno | job   |
+--------+-------+
|     20 | CLERK |
+--------+-------+
1 row in set (0.13 sec)

mysql> select * from emp where (deptno, job) = (select deptno, job from emp where ename = 'SMITH') and ename != 'SMITH';
+--------+-------+-------+------+---------------------+---------+------+--------+
| empno  | ename | job   | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+-------+------+---------------------+---------+------+--------+
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL |     20 |
+--------+-------+-------+------+---------------------+---------+------+--------+
1 row in set (0.14 sec)

-- 这里也可用in，这样就是多行，多列子查询了
select * from emp where (deptno, job) = (select deptno, job from emp where ename = 'SMITH') and ename != 'SMITH';

4.2 from后的子查询

子查询语句出现在from子句中，把一个子查询当做一个临时表使用。

显示每个高于自己部门平均工资的员工的姓名、部门、工资、平均工资

mysql> select deptno, avg(sal) avg from emp group by deptno;
+--------+-------------+
| deptno | avg         |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
|     30 | 1566.666667 |
+--------+-------------+
3 rows in set (0.11 sec)

mysql> select ename, t1.deptno, sal, avg from (select deptno, avg(sal) avg from emp group by deptno) t1, emp t2 where t1.deptno = t2.deptno and sal > avg;
+-------+--------+---------+-------------+
| ename | deptno | sal     | avg         |
+-------+--------+---------+-------------+
| ALLEN |     30 | 1600.00 | 1566.666667 |
| JONES |     20 | 2975.00 | 2175.000000 |
| BLAKE |     30 | 2850.00 | 1566.666667 |
| SCOTT |     20 | 3000.00 | 2175.000000 |
| KING  |     10 | 5000.00 | 2916.666667 |
| FORD  |     20 | 3000.00 | 2175.000000 |
+-------+--------+---------+-------------+
6 rows in set (0.10 sec)

-- 如果想要查询部门名称，需要再将上面的结果放到from子句后面，再套一层。不过嵌套太多了，可读性很差
mysql> select ename, t3.deptno, sal, avg, t4.dname from (select ename, t1.deptno, sal, avg from (select deptno, avg(sal) avg from emp group by deptno) t1, emp t2 where t1.deptno = t2.deptno and sal > avg) as t3, dept as t4 where t3.deptno = t4.deptno;
+-------+--------+---------+-------------+------------+
| ename | deptno | sal     | avg         | dname      |
+-------+--------+---------+-------------+------------+
| ALLEN |     30 | 1600.00 | 1566.666667 | SALES      |
| JONES |     20 | 2975.00 | 2175.000000 | RESEARCH   |
| BLAKE |     30 | 2850.00 | 1566.666667 | SALES      |
| SCOTT |     20 | 3000.00 | 2175.000000 | RESEARCH   |
| KING  |     10 | 5000.00 | 2916.666667 | ACCOUNTING |
| FORD  |     20 | 3000.00 | 2175.000000 | RESEARCH   |
+-------+--------+---------+-------------+------------+
6 rows in set (0.11 sec)

查找每个部门工资最高的人的姓名、工资、部门、最高工资

mysql> select max(sal) as max, deptno from emp group by deptno;
+---------+--------+
| max     | deptno |
+---------+--------+
| 5000.00 |     10 |
| 3000.00 |     20 |
| 2850.00 |     30 |
+---------+--------+
3 rows in set (0.12 sec)

mysql> select ename, sal, t1.deptno, max from emp as t1, (select max(sal) as max, deptno from emp group by deptno) as t2 where t1.deptno=t2.deptno and t1.sal = t2.max;
+-------+---------+--------+---------+
| ename | sal     | deptno | max     |
+-------+---------+--------+---------+
| BLAKE | 2850.00 |     30 | 2850.00 |
| SCOTT | 3000.00 |     20 | 3000.00 |
| KING  | 5000.00 |     10 | 5000.00 |
| FORD  | 3000.00 |     20 | 3000.00 |
+-------+---------+--------+---------+
4 rows in set (0.13 sec)

显示每个部门的信息（部门名，编号，地址）和人员数量

mysql> select t1.deptno, dname, loc, cnt from dept as t1, (select count(*) as cnt, deptno from emp group by deptno) as t2 where t1.deptno=t2.deptno;
+--------+------------+----------+-----+
| deptno | dname      | loc      | cnt |
+--------+------------+----------+-----+
|     10 | ACCOUNTING | NEW YORK |   3 |
|     20 | RESEARCH   | DALLAS   |   5 |
|     30 | SALES      | CHICAGO  |   6 |
+--------+------------+----------+-----+
3 rows in set (0.12 sec)

-- 使用多表查询，由于mysql的限制，所以group by后面要加上这些多余的限制条件
mysql> select dept.deptno, dname, loc, count(*) cnt from emp, dept where emp.deptno = dept.deptno group by dept.deptno, dname, loc;

4.3 合并查询

两个关键字: union和union all，union将select语句后的执行结果取并集，union all不做去重操作

将工资大于2500或职位是MANAGER的人找出来

mysql> select * from emp where sal > 2500 union select * from emp where job = 'MANGER';
+-------+-------+-----------+------+---------------------+---------+------+--------+
| empno | ename | job       | mgr  | hiredate            | sal     | comm | deptno |
+-------+-------+-----------+------+---------------------+---------+------+--------+
|  7566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
|  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 |
|  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |
|  7839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 |
|  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
+-------+-------+-----------+------+---------------------+---------+------+--------+
5 rows in set (0.11 sec)

mysql> select * from emp where sal > 2500 union all select * from emp where job = 'MANGER';
+-------+-------+-----------+------+---------------------+---------+------+--------+
| empno | ename | job       | mgr  | hiredate            | sal     | comm | deptno |
+-------+-------+-----------+------+---------------------+---------+------+--------+
|  7566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
|  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 |
|  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |
|  7839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 |
|  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
+-------+-------+-----------+------+---------------------+---------+------+--------+
5 rows in set (0.12 sec)

-- 注意: select 后面需要有相同的列
mysql> select * from emp where sal > 2500 union all select ename from emp where job = 'MANGER';
1222 - The used SELECT statements have a different number of columns

5. 内外连接

5.1 内连接

内连接可以看作是对笛卡尔积结果集的筛选。
从实现的角度来看，数据库在执行内连接时，可能会先计算两个表的笛卡尔积（在内部实现中，这可能是一种优化后的计算方式），然后根据连接条件（如ON子句指定的条件）来过滤掉不符合要求的行组合，最终得到内连接的结果。

语法：

select 字段 from 表1 inner join 表2 on 连接条件 and 其他条件；

显示SMITH的名字和部门名称

-- 使用前面笛卡尔积加筛选条件的写法
mysql> select ename, dname from emp, dept where emp.deptno=dept.deptno and emp.ename = 'SMITH';
+-------+----------+
| ename | dname    |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
1 row in set (0.13 sec)

-- 内连接
mysql> select ename, dname from emp inner join dept on emp.deptno = dept.deptno and ename = 'SMITH';
+-------+----------+
| ename | dname    |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
1 row in set (0.12 sec)
-- 注，上面的and也可以换成where，意味着先分好一个表后，再按where后的条件筛选

5.2 左外连接

返回左表（在FROM子句中位于LEFT JOIN关键字左边的表）中的所有行，以及右表（在LEFT JOIN关键字右边的表）中与左表连接条件匹配的行。如果右表中没有匹配的行，则在结果集中相应的列将填充为NULL值。

语法：

select 字段名 from 表名1 left join 表名2 on 连接条件

假设有下面两张表

mysql> select * from stu;
+----+------+
| id | name |
+----+------+
|  1 | jack |
|  2 | tom  |
|  3 | kity |
|  4 | nono |
+----+------+
4 rows in set (0.11 sec)

mysql> select * from exam;
+----+-------+
| id | grade |
+----+-------+
|  1 |    56 |
|  2 |    76 |
| 11 |     8 |
+----+-------+
3 rows in set (0.11 sec)

查询所有学生的成绩，如果这个学生没有成绩，也要将学生的个人信息显示出来

mysql> select * from stu left join exam on stu.id = exam.id;
+----+------+------+-------+
| id | name | id   | grade |
+----+------+------+-------+
|  1 | jack |    1 |    56 |
|  2 | tom  |    2 |    76 |
|  3 | kity | NULL | NULL  |
|  4 | nono | NULL | NULL  |
+----+------+------+-------+
4 rows in set (0.13 sec)

5.3 右外连接

返回右表（在FROM子句中位于RIGHT JOIN关键字右边的表）中的所有行，以及左表（在RIGHT JOIN关键字左边的表）中与右表连接条件匹配的行。如果左表中没有匹配的行，在结果集中相应的列会填充为NULL值。

语法：

select 字段名 from 表名1 right join 表名2 on 连接条件

继续使用5.3中的表

对stu表和exam表联合查询，把所有的成绩都显示出来，即使这个成绩没有学生与它对应，也要显示出来

mysql> select * from exam right join stu on exam.id = stu.id;
+------+-------+----+------+
| id   | grade | id | name |
+------+-------+----+------+
|    1 |    56 |  1 | jack |
|    2 |    76 |  2 | tom  |
| NULL | NULL  |  3 | kity |
| NULL | NULL  |  4 | nono |
+------+-------+----+------+
4 rows in set (0.11 sec)

使用emp表和dept表，列出部门名称和这些部门的员工信息，同时列出没有员工的部门

-- order by仅仅是为了方便观察
mysql> select emp.*, dept.dname from emp right join dept on dept.deptno = emp.deptno order by deptno asc;
+--------+--------+-----------+------+---------------------+---------+---------+--------+------------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno | dname      |
+--------+--------+-----------+------+---------------------+---------+---------+--------+------------+
| NULL   | NULL   | NULL      | NULL | NULL                | NULL    | NULL    | NULL   | OPERATIONS |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL    |     10 | ACCOUNTING |
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL    |     10 | ACCOUNTING |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL    |     10 | ACCOUNTING |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL    |     20 | RESEARCH   |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL    |     20 | RESEARCH   |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL    |     20 | RESEARCH   |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL    |     20 | RESEARCH   |
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 | 800.00  | NULL    |     20 | RESEARCH   |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 | SALES      |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00  |     30 | SALES      |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL    |     30 | SALES      |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00    |     30 | SALES      |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 | 950.00  | NULL    |     30 | SALES      |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00  |     30 | SALES      |
+--------+--------+-----------+------+---------------------+---------+---------+--------+------------+
15 rows in set (0.13 sec)