EMP表（Employee Table，员工表）
create table EMP(
empno int primary key,
ename varchar(50),
job varchar(50),
mgr int, ***ment "上级"
hiredate datetime, ***ment "入职日期"
sal decimal(10, 2), ***ment "薪水"
***m decimal(10, 2), ***ment "奖金"
deptno int
);
INSERT INTO EMP VALUES (7369, 'SMITH', 'CLERK', 7902, '1980-12-17 00:00:00', 800.00, NULL, 20);
INSERT INTO EMP VALUES (7499, 'ALLEN', 'SALESMAN', 7698, '1981-02-20 00:00:00', 1600.00, 300.00, 30);
INSERT INTO EMP VALUES (7521, 'WARD', 'SALESMAN', 7698, '1981-02-22 00:00:00', 1250.00, 500.00, 30);
INSERT INTO EMP VALUES (7566, 'JONES', 'MANAGER', 7839, '1981-04-02 00:00:00', 2975.00, NULL, 20);
INSERT INTO EMP VALUES (7654, 'MARTIN', 'SALESMAN', 7698, '1981-09-28 00:00:00', 1250.00, 1400.00, 30);
INSERT INTO EMP VALUES (7698, 'BLAKE', 'MANAGER', 7839, '1981-05-01 00:00:00', 2850.00, NULL, 30);
INSERT INTO EMP VALUES (7782, 'CLARK', 'MANAGER', 7839, '1981-06-09 00:00:00', 2450.00, NULL, 10);
INSERT INTO EMP VALUES (7788, 'SCOTT', 'ANALYST', 7566, '1987-04-19 00:00:00', 3000.00, NULL, 20);
INSERT INTO EMP VALUES (7839, 'KING', 'PRESIDENT', NULL, '1981-11-17 00:00:00', 5000.00, NULL, 10);
INSERT INTO EMP VALUES (7844, 'TURNER', 'SALESMAN', 7698, '1981-09-08 00:00:00', 1500.00, 0.00, 30);
INSERT INTO EMP VALUES (7876, 'ADAMS', 'CLERK', 7788, '1987-05-23 00:00:00', 1100.00, NULL, 20);
INSERT INTO EMP VALUES (7900, 'JAMES', 'CLERK', 7698, '1981-12-03 00:00:00', 950.00, NULL, 30);
INSERT INTO EMP VALUES (7902, 'FORD', 'ANALYST', 7566, '1981-12-03 00:00:00', 3000.00, NULL, 20);
INSERT INTO EMP VALUES (7934, 'MILLER', 'CLERK', 7782, '1982-01-23 00:00:00', 1300.00, NULL, 10);
DEPT表（Department Table，部门表）
create table DEPT(
deptno int,
dname varchar(20),
loc varchar(20));
INSERT INTO DEPT (deptno, dname, loc) VALUES (10, 'A***OUNTING', 'NEW YORK');
INSERT INTO DEPT (deptno, dname, loc) VALUES (20, 'RESEARCH', 'DALLAS');
INSERT INTO DEPT (deptno, dname, loc) VALUES (30, 'SALES', 'CHICAGO');
INSERT INTO DEPT (deptno, dname, loc) VALUES (40, 'OPERATIONS', 'BOSTON');
多表查询是指在一个 SQL 查询语句中，从两个或两个以上的表中获取数据。在关系型数据库中，表与表之间通常通过外键等关联关系连接，多表查询可以将这些关联表中的数据结合起来，满足更复杂的查询需求。常见的多表查询方式有以下几种：
1.交叉连接
交叉连接会返回两个表中所有行的组合，也称为笛卡尔积。如果表 A 有 m 行，表 B 有 n 行，那么交叉连接的结果会有 m×n 行。
显示雇员名、雇员工资以及所在部门的名字因为上面的数据来自EMP和DEPT表，因此要联合查询
其实我们只要emp表中的deptno = dept表中的deptno字段的记录
mysql> select EMP.ename, EMP.sal, DEPT.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 | A***OUNTING |
| SCOTT | 3000.00 | RESEARCH |
| KING | 5000.00 | A***OUNTING |
| TURNER | 1500.00 | SALES |
| ADAMS | 1100.00 | RESEARCH |
| JAMES | 950.00 | SALES |
| FORD | 3000.00 | RESEARCH |
| MILLER | 1300.00 | A***OUNTING |
+--------+---------+------------+
14 rows in set (0.00 sec)
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 |
+------------+--------+---------+
| A***OUNTING | CLARK | 2450.00 |
| A***OUNTING | KING | 5000.00 |
| A***OUNTING | MILLER | 1300.00 |
+------------+--------+---------+
2.自连接
自连接是指一个表与自身进行连接，通常用于处理表中存在层次关系或自身关联的情况。
显示员工FORD的上级领导的编号和姓名
使用子查询
mysql> select empno, ename from EMP
-> where mgr = (select empno from EMP where ename='FORD');
+-------+-------+
| empno | ename |
+-------+-------+
| 7369 | SMITH |
+-------+-------+
1 row in set (0.00 sec)
使用多表查询
mysql> select leader.empno, leader.ename from EMP leader, EMP worker
-> where worker.ename = "FORD" and leader.empno = worker.empno;
+-------+-------+
| empno | ename |
+-------+-------+
| 7902 | FORD |
+-------+-------+
1 row in set (0.00 sec)
3.子查询
子查询是指嵌入在其他sql语句中的select语句，也叫嵌套查询
3.1.单行子查询
返回一行记录的子查询
显示SMITH同一部门的员工
mysql> select * from EMP where deptno = (select deptno from EMP where ename = 'SMITH');
+-------+-------+---------+------+---------------------+---------+------+--------+
| empno | ename | job | mgr | hiredate | sal | ***m | deptno |
+-------+-------+---------+------+---------------------+---------+------+--------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 |
| 7876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 |
+-------+-------+---------+------+---------------------+---------+------+--------+
5 rows in set (0.00 sec)
只返回 deptno 这 1 列，且结果是 20（只有 1 行），属于单行子查询。
3.2.多行子查询
返回多行记录的子查询
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。
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 (0.01 sec)
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 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 (0.00 sec)
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 (0.00 sec)
只返回 deptno 这 1 列，但结果可能有 10、20、30 多行（多个部门编号），属于多行子查询。
3.3.多列子查询
单行子查询是指子查询只返回单列，单行数据；多行子查询是指返回单列多行数据，都是针对单列而言
的，而多列子查询则是指查询返回多个列数据的子查询语句
案例：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 * from EMP where
-> (deptno,job)=(select deptno, job from EMP where ename="SMITH")
-> and ename <> 'SMITH';
+-------+-------+-------+------+---------------------+---------+------+--------+
| empno | ename | job | mgr | hiredate | sal | ***m | deptno |
+-------+-------+-------+------+---------------------+---------+------+--------+
| 7876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 |
+-------+-------+-------+------+---------------------+---------+------+--------+
1 row in set (0.00 sec)
3.4.在from中使用子查询
子查询语句出现在from子句中。这里要用到数据查询的技巧，把一个子查询当做一个临时表使用。
案例：
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 EMP.ename, EMP.deptno, sal
-> from EMP, (select deptno, avg(sal) asal from EMP group by deptno) tmp
-> where EMP.sal > tmp.asal and EMP.deptno=tmp.deptno;
+-------+--------+---------+
| ename | deptno | sal |
+-------+--------+---------+
| ALLEN | 30 | 1600.00 |
| JONES | 20 | 2975.00 |
| BLAKE | 30 | 2850.00 |
| SCOTT | 20 | 3000.00 |
| KING | 10 | 5000.00 |
| FORD | 20 | 3000.00 |
+-------+--------+---------+
6 rows in set (0.00 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 EMP.ename, EMP.sal, EMP.deptno, ms
-> from EMP, (select max(sal) ms, deptno from EMP group by deptno) tmp
-> where EMP.deptno=tmp.deptno and EMP.sal=tmp.ms;
+-------+---------+--------+---------+
| ename | sal | deptno | ms |
+-------+---------+--------+---------+
| 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 (0.00 sec)
显示每个部门的信息（部门名，编号，地址）和人员数量
方法1：使用多表
mysql> select DEPT.dname, DEPT.deptno, DEPT.loc, count(*)
-> from DEPT, EMP where DEPT.deptno=EMP.deptno
-> group by DEPT.dname, DEPT.deptno, DEPT.loc;
+------------+--------+----------+----------+
| dname | deptno | loc | count(*) |
+------------+--------+----------+----------+
| A***OUNTING | 10 | NEW YORK | 3 |
| RESEARCH | 20 | DALLAS | 5 |
| SALES | 30 | CHICAGO | 6 |
+------------+--------+----------+----------+
3 rows in set (0.00 sec)
方法2：使用子查询
mysql> select DEPT.dname, DEPT.deptno, DEPT.loc, num
-> from DEPT, (select deptno, count(*) num from EMP group by deptno) tmp
-> where DEPT.deptno=tmp.deptno;
+------------+--------+----------+-----+
| dname | deptno | loc | num |
+------------+--------+----------+-----+
| A***OUNTING | 10 | NEW YORK | 3 |
| RESEARCH | 20 | DALLAS | 5 |
| SALES | 30 | CHICAGO | 6 |
+------------+--------+----------+-----+
3 rows in set (0.00 sec)
3.5.合并查询
在实际应用中，为了合并多个select的执行结果，可以使用集合操作符 union，union all
3.5.1.union
该操作符用于取得两个结果集的并集。当使用该操作符时，会自动去掉结果集中的重复行。
案例：将工资大于2500或职位是MANAGER的人找出来
mysql> select ename, sal, job from EMP where sal>2500 union
-> select ename, sal, job from EMP where job='MANAGER';
+-------+---------+-----------+
| ename | sal | job |
+-------+---------+-----------+
| JONES | 2975.00 | MANAGER |
| BLAKE | 2850.00 | MANAGER |
| SCOTT | 3000.00 | ANALYST |
| KING | 5000.00 | PRESIDENT |
| FORD | 3000.00 | ANALYST |
| CLARK | 2450.00 | MANAGER |
+-------+---------+-----------+
6 rows in set (0.00 sec)
3.5.2.union all
该操作符用于取得两个结果集的并集。当使用该操作符时，不会去掉结果集中的重复行。
mysql> select ename, sal, job from EMP where sal>2500 union all
-> select ename, sal, job from EMP where job='MANAGER';
+-------+---------+-----------+
| ename | sal | job |
+-------+---------+-----------+
| JONES | 2975.00 | MANAGER |
| BLAKE | 2850.00 | MANAGER |
| SCOTT | 3000.00 | ANALYST |
| KING | 5000.00 | PRESIDENT |
| FORD | 3000.00 | ANALYST |
| JONES | 2975.00 | MANAGER |
| BLAKE | 2850.00 | MANAGER |
| CLARK | 2450.00 | MANAGER |
+-------+---------+-----------+
8 rows in set (0.00 sec)
4.内连接和外连接
4.1.内连接
在数据库查询中，内连接（INNER JOIN）和外连接（OUTER JOIN）是用于关联多个表时的两种常用连接方式，它们的主要区别在于如何处理不匹配的记录。
内连接只返回两个表中匹配条件的记录，即只包含在两个表中都存在对应关系的数据。内连接实际上就是利用 where 子句对两种表形成的笛卡儿积进行筛选，前面学习的查询都是内连接，也是在开发过程中使用的最多的连接查询。
select 字段 from 表1 inner join 表2 on 连接条件 and 其他条件；
案例
-- 用前面的写法
select ename, dname from EMP, DEPT where EMP.deptno=DEPT.deptno
and ename='SMITH';
-- 用标准的内连接写法
select ename, dname from EMP inner join DEPT on EMP.deptno=DEPT.deptno
and ename='SMITH';
4.2.外连接
外连接会返回至少一个表中满足连接条件的记录，根据保留哪个表的不匹配记录，又分为：左外连接和右外连接。
4.2.1.左外连接
返回左表中的所有记录，以及右表中匹配条件的记录。如果右表中没有匹配项，则显示 NULL。
select 字段名 from 表名1 left join 表名2 on 连接条件
create table stu (id int, name varchar(30)); -- 学生表
insert into stu values(1,'jack'),(2,'tom'),(3,'kity'),(4,'nono');
create table exam (id int, grade int); -- 成绩表
insert into exam values(1, 56),(2,76),(11, 8);
mysql> select * from stu;
+------+------+
| id | name |
+------+------+
| 1 | jack |
| 2 | tom |
| 3 | kity |
| 4 | nono |
+------+------+
mysql> select * from exam;
+------+-------+
| id | grade |
+------+-------+
| 1 | 56 |
| 2 | 76 |
| 11 | 8 |
+------+-------+
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 (0.00 sec)
4.2.2.右外连接
返回右表中的所有记录，以及左表中匹配条件的记录。如果左表中没有匹配项，则显示 NULL。
select 字段 from 表名1 right join 表名2 on 连接条件
mysql> select * from stu right join exam on stu.id=exam.id;
+------+------+------+-------+
| id | name | id | grade |
+------+------+------+-------+
| 1 | jack | 1 | 56 |
| 2 | tom | 2 | 76 |
| NULL | NULL | 11 | 8 |
+------+------+------+-------+
3 rows in set (0.00 sec)
