（一）连接查询？
从一张表中单独查询，称为单表查询。
emp表和dept表联合起来进行查询数据，从emp表中取员工名字，从dept表中取部门名字，这种跨表查询，多张表联合起来进行查询数据的方式称为连接查询。
（二）分类
标准一：语法年代
SQL92：1992年出现的语法
SQL99：1999年出现的语法
标准二：连接方式
内连接：
① 等值连接
② 非等值连接
③ 自连接
外连接：
① 左外连接（左连接）
② 右外连接（右连接）
全连接：
笛卡尔积现象
两张表进行连接查询，没有任何条件限制的时候，追踪查询的条数，是两张表条数的乘积，此现象被称为“笛卡尔积现象”
实例如下：
mysql> select dname from dept;
+------------+
| dname |
+------------+
| ACCOUNTING |
| RESEARCH |
| SALES |
| OPERATIONS |
+------------+
4 rows in set (0.00 sec)
mysql> select dname,deptno from dept;
+------------+--------+
| dname | deptno |
+------------+--------+
| ACCOUNTING | 10 |
| RESEARCH | 20 |
| SALES | 30 |
| OPERATIONS | 40 |
+------------+--------+
4 rows in set (0.00 sec)
#从从两个表中各取一个字段
mysql> select ename,dname from emp,dept;
+--------+------------+
| ename | dname |
+--------+------------+
| SMITH | OPERATIONS |
| SMITH | SALES |
| SMITH | RESEARCH |
| SMITH | ACCOUNTING |
| ALLEN | OPERATIONS |
| ALLEN | SALES |
| ALLEN | RESEARCH |
| ALLEN | ACCOUNTING |
| WARD | OPERATIONS |
| WARD | SALES |
| WARD | RESEARCH |
| WARD | ACCOUNTING |
| JONES | OPERATIONS |
| JONES | SALES |
| JONES | RESEARCH |
| JONES | ACCOUNTING |
| MARTIN | OPERATIONS |
| MARTIN | SALES |
| MARTIN | RESEARCH |
| MARTIN | ACCOUNTING |
| BLAKE | OPERATIONS |
| BLAKE | SALES |
| BLAKE | RESEARCH |
| BLAKE | ACCOUNTING |
| CLARK | OPERATIONS |
| CLARK | SALES |
| CLARK | RESEARCH |
| CLARK | ACCOUNTING |
| SCOTT | OPERATIONS |
| SCOTT | SALES |
| SCOTT | RESEARCH |
| SCOTT | ACCOUNTING |
| KING | OPERATIONS |
| KING | SALES |
| KING | RESEARCH |
| KING | ACCOUNTING |
| TURNER | OPERATIONS |
| TURNER | SALES |
| TURNER | RESEARCH |
| TURNER | ACCOUNTING |
| ADAMS | OPERATIONS |
| ADAMS | SALES |
| ADAMS | RESEARCH |
| ADAMS | ACCOUNTING |
| JAMES | OPERATIONS |
| JAMES | SALES |
| JAMES | RESEARCH |
| JAMES | ACCOUNTING |
| FORD | OPERATIONS |
| FORD | SALES |
| FORD | RESEARCH |
| FORD | ACCOUNTING |
| MILLER | OPERATIONS |
| MILLER | SALES |
| MILLER | RESEARCH |
| MILLER | ACCOUNTING |
+--------+------------+
56 rows in set (0.00 sec)
补充：匹配过程是将一个中一个所有记录和另一个表中的数据进行一一匹配
解决方案：
连接时添加条件，满足条件可以将所需要的纪录被筛选出来
条件：通过将两张表中相同的记录进行比较
mysql> select ename,dname from emp,dept where emp.deptno = dept.deptno;
+--------+------------+
| 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)
#给表起别名，提升效率（SQL92语法）
mysql> select e.ename,d.dname from emp e,dept d where e.deptno = d.deptno;
+--------+------------+
| 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)
说明：查询的结果变成了14条，但是匹配的次数还是56次，并没有减少。
结论：通过笛卡尔积现象可以得出，表的连接次数越多效率越低，尽量避免表的连接次数。
（三）查询方式
I.内连接
特点：将匹配中满足条件的数据查询出来(两个表之间没有主次关系)
（1）等值连接
实例：查询每个员工所在部门名称，显示员工名和部门名？
#SQL92语法
mysql> select e.ename,d.dname from emp e,dept d where e.deptno = d.deptno;
+--------+------------+
| 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)
#SQL99（说明：inner是可以省略的）
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 |
| 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)
总结：
① SQL92的缺点：
结构不清晰，表的连接条件和后期进一步筛选的条件，均放到了where的后面。
② SQL99（常用）的优点：
表连接条件独立，连接之后，对于进一步的筛选，可以将条件添加到where后。
SQL99的语法：
select ...
from a
join b
on a和b的连接条件；
（2）非等值连接
说明：指的是条件不是一个等量关系，称为非等值连接
案例：找出每个员工的薪资等级，要求显示员工名、薪资、薪资等级。
#查询员工基本信息表
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)
#查询薪资等级表
mysql> select * from salgrade;
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
| 1 | 700 | 1200 |
| 2 | 1201 | 1400 |
| 3 | 1401 | 2000 |
| 4 | 2001 | 3000 |
| 5 | 3001 | 9999 |
+-------+-------+-------+
5 rows in set (0.00 sec)
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 (0.00 sec)
（3）自连接
说明：一张表看作两张表
案例： 查询员工的上级领导，要求显示员工名和对应的领导名？
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 empno,ename,mgr from emp;
+-------+--------+------+
| empno | ename | mgr |
+-------+--------+------+
| 7369 | SMITH | 7902 |
| 7499 | ALLEN | 7698 |
| 7521 | WARD | 7698 |
| 7566 | JONES | 7839 |
| 7654 | MARTIN | 7698 |
| 7698 | BLAKE | 7839 |
| 7782 | CLARK | 7839 |
| 7788 | SCOTT | 7566 |
| 7839 | KING | NULL |
| 7844 | TURNER | 7698 |
| 7876 | ADAMS | 7788 |
| 7900 | JAMES | 7698 |
| 7902 | FORD | 7566 |
| 7934 | MILLER | 7782 |
+-------+--------+------+
14 rows in set (0.00 sec)
#技巧：将一张表看成是两张表 一张是员工表 一张是领导表
#员工表 emp a
+-------+--------+------+
| empno | ename | mgr |
+-------+--------+------+
| 7369 | SMITH | 7902 |
| 7499 | ALLEN | 7698 |
| 7521 | WARD | 7698 |
| 7566 | JONES | 7839 |
| 7654 | MARTIN | 7698 |
| 7698 | BLAKE | 7839 |
| 7782 | CLARK | 7839 |
| 7788 | SCOTT | 7566 |
| 7839 | KING | NULL |
| 7844 | TURNER | 7698 |
| 7876 | ADAMS | 7788 |
| 7900 | JAMES | 7698 |
| 7902 | FORD | 7566 |
| 7934 | MILLER | 7782 |
+-------+--------+------+
#领导表 emp b
+-------+--------+------+
| empno | ename | mgr |
+-------+--------+------+
| 7369 | SMITH | 7902 |
| 7499 | ALLEN | 7698 |
| 7521 | WARD | 7698 |
| 7566 | JONES | 7839 |
| 7654 | MARTIN | 7698 |
| 7698 | BLAKE | 7839 |
| 7782 | CLARK | 7839 |
| 7788 | SCOTT | 7566 |
| 7839 | KING | NULL |
| 7844 | TURNER | 7698 |
| 7876 | ADAMS | 7788 |
| 7900 | JAMES | 7698 |
| 7902 | FORD | 7566 |
| 7934 | MILLER | 7782 |
+-------+--------+------+
#将第一表中员工的领导编号和第二个表中的编号进行匹配找到对应的领导名字
mysql> select a.ename'员工名',b.ename '领导名' from emp a join emp b on a.mgr=b.empno;
+--------+--------+
| 员工名 | 领导名 |
+--------+--------+
| 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)
II.外连接
特点：两个表之间存在主次关系
案例：查询员工的上级领导，要求显示员工名和对应的领导名？
说明：要将能匹配上的和不能匹配上的数据都打印出来
#right作用：将join关键字右边的这张表看作是主表，主要是为了将这张表的数据全部查询出来，
# 捎带着关联查询左边的表，在外连接中，两张表连接，产生了主次关系
# outer是可以省略的，带着的话可以增加可读性
mysql> select e.ename ,d.dname from emp e right outer 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 | #未匹配成功的
+--------+------------+
15 rows in set (0.00 sec)
#right：表示的是其左边的这个表是主表，右边的表是捎带表
mysql> select e.ename ,d.dname from dept d left outer 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 | #未匹配成功的
+--------+------------+
15 rows in set (0.00 sec)
说明：
①带有right是右外连接，又叫做右连接
②带有left是左外连接，又叫做左连接
③任何一左连接都有右连接的写法，任何一个右连接都有左连接的写法
内连接 VS 外连接
区分： 带有关键字‘ left ’或‘ right ’的连接一定是外连接。
查询条数：
外连接的查询结果一定是大于内连接的查询结果条数。
实例：查询每个员工的上级领导，要求显示所有员工的名字和领导名？？
mysql> select empno ,ename,mgr from emp;
+-------+--------+------+
| empno | ename | mgr |
+-------+--------+------+
| 7369 | SMITH | 7902 |
| 7499 | ALLEN | 7698 |
| 7521 | WARD | 7698 |
| 7566 | JONES | 7839 |
| 7654 | MARTIN | 7698 |
| 7698 | BLAKE | 7839 |
| 7782 | CLARK | 7839 |
| 7788 | SCOTT | 7566 |
| 7839 | KING | NULL |
| 7844 | TURNER | 7698 |
| 7876 | ADAMS | 7788 |
| 7900 | JAMES | 7698 |
| 7902 | FORD | 7566 |
| 7934 | MILLER | 7782 |
+-------+--------+------+
14 rows in set (0.00 sec）
#内连接
mysql> select a.ename'员工名',b.ename '领导名' from emp a join emp b on a.mgr=b.empno;
+--------+--------+
| 员工名 | 领导名 |
+--------+--------+
| 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)
#外连接
mysql> select a.ename'员工名',b.ename '领导名' from emp a left join emp b on a.mgr=b.empno;
+--------+--------+
| 员工名 | 领导名 |
+--------+--------+
| 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)
拓展：多表连接
语法：
select
...
from
a
join
b
on
a 与 b连接的条件
join
c
on
a 与 c连接的条件
right join
d
on
a 与 d连接的条件
一条SQL语句 中内连接和外连接可以混合
实例：
找出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？
#员工表
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)
#部门表
mysql> select * from dept;
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
4 rows in set (0.00 sec)
#薪资等级表
mysql> select * from salgrade;
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
| 1 | 700 | 1200 |
| 2 | 1201 | 1400 |
| 3 | 1401 | 2000 |
| 4 | 2001 | 3000 |
| 5 | 3001 | 9999 |
+-------+-------+-------+
5 rows in set (0.00 sec)
mysql> select e.ename,e.sal,d.dname,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 | sal | dname | grade |
+--------+---------+------------+-------+
| SMITH | 800.00 | RESEARCH | 1 |
| ALLEN | 1600.00 | SALES | 3 |
| WARD | 1250.00 | SALES | 2 |
| JONES | 2975.00 | RESEARCH | 4 |
| MARTIN | 1250.00 | SALES | 2 |
| BLAKE | 2850.00 | SALES | 4 |
| CLARK | 2450.00 | ACCOUNTING | 4 |
| SCOTT | 3000.00 | RESEARCH | 4 |
| KING | 5000.00 | ACCOUNTING | 5 |
| TURNER | 1500.00 | SALES | 3 |
| ADAMS | 1100.00 | RESEARCH | 1 |
| JAMES | 950.00 | SALES | 1 |
| FORD | 3000.00 | RESEARCH | 4 |
| MILLER | 1300.00 | ACCOUNTING | 2 |
+--------+---------+------------+-------+
14 rows in set (0.00 sec)
改进：添加输出领导名
#内连接
mysql> select e.ename,e.sal,d.dname,s.grade,l.ename from emp e join dept d on e.deptno=d.deptno join salgrade s on e.sal between s.losal and s.hisal join emp l on e.mgr=l.empno;
+--------+---------+------------+-------+-------+
| ename | sal | dname | grade | ename |
+--------+---------+------------+-------+-------+
| SMITH | 800.00 | RESEARCH | 1 | FORD |
| ALLEN | 1600.00 | SALES | 3 | BLAKE |
| WARD | 1250.00 | SALES | 2 | BLAKE |
| JONES | 2975.00 | RESEARCH | 4 | KING |
| MARTIN | 1250.00 | SALES | 2 | BLAKE |
| BLAKE | 2850.00 | SALES | 4 | KING |
| CLARK | 2450.00 | ACCOUNTING | 4 | KING |
| SCOTT | 3000.00 | RESEARCH | 4 | JONES |
| TURNER | 1500.00 | SALES | 3 | BLAKE |
| ADAMS | 1100.00 | RESEARCH | 1 | SCOTT |
| JAMES | 950.00 | SALES | 1 | BLAKE |
| FORD | 3000.00 | RESEARCH | 4 | JONES |
| MILLER | 1300.00 | ACCOUNTING | 2 | CLARK |
+--------+---------+------------+-------+-------+
13 rows in set (0.00 sec)
#使用左（外）连接
mysql> select e.ename,e.sal,d.dname,s.grade,l.ename from emp e join dept d on e.deptno=d.deptno join salgrade s on e.sal between s.losal and s.hisal left join emp l on e.mgr=l.empno;
+--------+---------+------------+-------+-------+
| ename | sal | dname | grade | ename |
+--------+---------+------------+-------+-------+
| SMITH | 800.00 | RESEARCH | 1 | FORD |
| ALLEN | 1600.00 | SALES | 3 | BLAKE |
| WARD | 1250.00 | SALES | 2 | BLAKE |
| JONES | 2975.00 | RESEARCH | 4 | KING |
| MARTIN | 1250.00 | SALES | 2 | BLAKE |
| BLAKE | 2850.00 | SALES | 4 | KING |
| CLARK | 2450.00 | ACCOUNTING | 4 | KING |
| SCOTT | 3000.00 | RESEARCH | 4 | JONES |
| KING | 5000.00 | ACCOUNTING | 5 | NULL |
| TURNER | 1500.00 | SALES | 3 | BLAKE |
| ADAMS | 1100.00 | RESEARCH | 1 | SCOTT |
| JAMES | 950.00 | SALES | 1 | BLAKE |
| FORD | 3000.00 | RESEARCH | 4 | JONES |
| MILLER | 1300.00 | ACCOUNTING | 2 | CLARK |
+--------+---------+------------+-------+-------+
14 rows in set (0.00 sec)
#起别名后的薪资表
mysql> select e.ename'员工',e.sal'薪水',d.dname'部门名',s.grade'薪水等级',l.ename'领导' from emp e join dept d on e.deptno=d.deptno join salgrade s on e.sal between s.losal and s.hisal left join emp l on e.mgr=l.empno;
+--------+---------+------------+----------+-------+
| 员工 | 薪水 | 部门名 | 薪水等级 | 领导 |
+--------+---------+------------+----------+-------+
| SMITH | 800.00 | RESEARCH | 1 | FORD |
| ALLEN | 1600.00 | SALES | 3 | BLAKE |
| WARD | 1250.00 | SALES | 2 | BLAKE |
| JONES | 2975.00 | RESEARCH | 4 | KING |
| MARTIN | 1250.00 | SALES | 2 | BLAKE |
| BLAKE | 2850.00 | SALES | 4 | KING |
| CLARK | 2450.00 | ACCOUNTING | 4 | KING |
| SCOTT | 3000.00 | RESEARCH | 4 | JONES |
| KING | 5000.00 | ACCOUNTING | 5 | NULL |
| TURNER | 1500.00 | SALES | 3 | BLAKE |
| ADAMS | 1100.00 | RESEARCH | 1 | SCOTT |
| JAMES | 950.00 | SALES | 1 | BLAKE |
| FORD | 3000.00 | RESEARCH | 4 | JONES |
| MILLER | 1300.00 | ACCOUNTING | 2 | CLARK |
+--------+---------+------------+----------+-------+
14 rows in set (0.00 sec)