多表连接

语法：
select
……
from
a
join
b
on
a和b的连接条件
join
c
on
a和c的连接条件
表示：A表和B表先进行表连接，连接之后A表继续和C表进行连接。
一个SQL中内连接和外连接可以混合

案例：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
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 losal and hisal order by e.sal;
                +--------+------------+---------+-------+
                | ename  | dname      | sal     | grade |
                +--------+------------+---------+-------+
                | SMITH  | RESEARCH   |  800.00 |     1 |
                | JAMES  | SALES      |  950.00 |     1 |
                | ADAMS  | RESEARCH   | 1100.00 |     1 |
                | WARD   | SALES      | 1250.00 |     2 |
                | MARTIN | SALES      | 1250.00 |     2 |
                | MILLER | ACCOUNTING | 1300.00 |     2 |
                | TURNER | SALES      | 1500.00 |     3 |
                | ALLEN  | SALES      | 1600.00 |     3 |
                | CLARK  | ACCOUNTING | 2450.00 |     4 |
                | BLAKE  | SALES      | 2850.00 |     4 |
                | JONES  | RESEARCH   | 2975.00 |     4 |
                | SCOTT  | RESEARCH   | 3000.00 |     4 |
                | FORD   | RESEARCH   | 3000.00 |     4 |
                | KING   | ACCOUNTING | 5000.00 |     5 |
                +--------+------------+---------+-------+
案例：找出每个员工的部门名称以及工资等级、还有上级领导，
要求显示员工名、领导名、部门名、薪资、薪资等级
select a.ename '员工名',b.ename '领导名',d.dname,a.sal,s.grade from emp a join dept d on a.deptno = d.deptno join salgrade s on a.sal between losal and hisal left join emp b on a.mgr = b.empno order by a.sal;
            +--------+--------+------------+---------+-------+
            | 员工名 | 领导名 | dname      | sal     | grade |
            +--------+--------+------------+---------+-------+
            | SMITH  | FORD   | RESEARCH   |  800.00 |     1 |
            | JAMES  | BLAKE  | SALES      |  950.00 |     1 |
            | ADAMS  | SCOTT  | RESEARCH   | 1100.00 |     1 |
            | MARTIN | BLAKE  | SALES      | 1250.00 |     2 |
            | WARD   | BLAKE  | SALES      | 1250.00 |     2 |
            | MILLER | CLARK  | ACCOUNTING | 1300.00 |     2 |
            | TURNER | BLAKE  | SALES      | 1500.00 |     3 |
            | ALLEN  | BLAKE  | SALES      | 1600.00 |     3 |
            | CLARK  | KING   | ACCOUNTING | 2450.00 |     4 |
            | BLAKE  | KING   | SALES      | 2850.00 |     4 |
            | JONES  | KING   | RESEARCH   | 2975.00 |     4 |
            | FORD   | JONES  | RESEARCH   | 3000.00 |     4 |
            | SCOTT  | JONES  | RESEARCH   | 3000.00 |     4 |
            | KING   | NULL   | ACCOUNTING | 5000.00 |     5 |
            +--------+--------+------------+---------+-------+