多表连接

• 使用join on 语句
  select emp.ename,dept.dname
  from emp
  join dept on dept.deptno = emp.deptno
  join [表名] on [[表名]与dept的连接条件]

• 左外连接
  select emp.ename,dept.dname
  from dept
  left join emp on dept.deptno = emp.deptno
  左边表的所有行都会出现在结果中

• 自身连接
  select e.ename,f.ename
  from emp e
  join emp f on e.mgr = f.empno