以scott emp表为例
select a.ename as employee_name,b.ename as boss_name from emp a,emp b where a.mgr = b.empno;
思路
1 确定自联,查询后发现是一个笛卡尔积
分析后发现,雇员和老板的对应关系是 7369 SMITH 7902 FORD
2 根据上述分析,将约束条件加上
select a.empno,a.ename,a.mgr,b.empno,b.ename,b.mgr from emp a,emp b where a.mgr = b.empno;
3 筛选投影后取别名
select a.ename as employee_name,b.ename as boss_name from emp a,emp b where a.mgr = b.empno;
EMPLOYEE_N BOSS_NAME
---------- ----------
FORD JONES
SCOTT JONES
TURNER BLAKE
ALLEN BLAKE
WARD BLAKE
JAMES BLAKE
MARTIN BLAKE
MILLER CLARK
ADAMS SCOTT
BLAKE KING
JONES KING
CLARK KING
SMITH FORD
13 rows selected.