备注:测试数据库版本为MySQL 8.0
如需要scott用户下建表及录入数据语句，可参考:
scott建表及录入数据sql脚本
一.需求
返回一个结果集，它描述整个表的层次。
在EMP表中，员工KING没有经理，所以KING是根节点。
从KING开始，显示KING下面的所有员工以及KING下属的所有员工(如存在)。
最后，返回下列结果集:
+------------------------------+
| emp_tree |
+------------------------------+
| KING |
| KING - BLAKE |
| KING - BLAKE - ALLEN |
| KING - BLAKE - JAMES |
| KING - BLAKE - MARTIN |
| KING - BLAKE - TURNER |
| KING - BLAKE - WARD |
| KING - CLARK |
| KING - CLARK - MILLER |
| KING - JONES |
| KING - JONES - FORD |
| KING - JONES - FORD - SMITH |
| KING - JONES - SCOTT |
| KING - JONES - SCOTT - ADAMS |
+------------------------------+
二.解决方案
2.1 使用union和多个自联接
select emp_tree
from (
select ename as emp_tree
from emp
where mgr is null
union
select concat(a.ename,' - ',b.ename)
from emp a
inner join emp b
on a.empno = b.mgr
where a.mgr is null
union
select concat(a.ename,' - ',
b.ename,' - ',c.ename)
from emp a
inner join emp b
on a.empno = b.mgr
left join emp c
on b.empno = c.mgr
where a.ename = 'KING'
union
select concat(a.ename,' - ',b.ename,' - ',
c.ename,' - ',d.ename)
from emp a
inner join emp b
on a.emono = b.mgr
inner join emp c
on b.emono = c.mgr
left join emp d
on c.empno = d.mgr
where a.ename = 'KING'
） x
where tree is not null
order by 1;
测试记录:
mysql> select emp_tree
-> from (
-> select ename as emp_tree
-> from emp
-> where mgr is null
-> union
-> select concat(a.ename,' - ',b.ename)
-> from emp a
-> inner join emp b
-> on a.empno = b.mgr
-> where a.mgr is null
-> union
-> select concat(a.ename,' - ',
-> b.ename,' - ',c.ename)
-> from emp a
-> inner join emp b
-> on a.empno = b.mgr
-> left join emp c
-> on b.empno = c.mgr
-> where a.ename = 'KING'
-> union
-> select concat(a.ename,' - ',b.ename,' - ',
-> c.ename,' - ',d.ename)
-> from emp a
-> inner join emp b
-> on a.empno = b.mgr
-> inner join emp c
-> on b.empno = c.mgr
-> left join emp d
-> on c.empno = d.mgr
-> where a.ename = 'KING'
-> ) x
-> where emp_tree is not null
-> order by 1;
+------------------------------+
| emp_tree |
+------------------------------+
| KING |
| KING - BLAKE |
| KING - BLAKE - ALLEN |
| KING - BLAKE - JAMES |
| KING - BLAKE - MARTIN |
| KING - BLAKE - TURNER |
| KING - BLAKE - WARD |
| KING - CLARK |
| KING - CLARK - MILLER |
| KING - JONES |
| KING - JONES - FORD |
| KING - JONES - FORD - SMITH |
| KING - JONES - SCOTT |
| KING - JONES - SCOTT - ADAMS |
+------------------------------+
14 rows in set (0.00 sec)
2.2 with递归方法
可以看到MySQL 8.0开始支持的with递归，可以让代码大大简便，代码逻辑看起来也更有层次。
with recursive emp2(ename,empno) AS
(
SELECT cast(ename as char(200)) ename,empno
from emp
where mgr is null
union ALL
SELECT concat(e2.ename,' - ',e1.ename) ,e1.empno
from emp e1,emp2 e2
where e1.mgr = e2.empno
)
select trim(ename) as ename
from emp2
测试记录
mysql> with recursive emp2(ename,empno) AS
-> (
-> SELECT cast(ename as char(200)) ename,empno
-> from emp
-> where mgr is null
-> union ALL
-> SELECT concat(e2.ename,' - ',e1.ename) ,e1.empno
-> from emp e1,emp2 e2
-> where e1.mgr = e2.empno
-> )
-> select trim(ename) as ename
-> from emp2;
+------------------------------+
| ename |
+------------------------------+
| KING |
| KING - JONES |
| KING - BLAKE |
| KING - CLARK |
| KING - JONES - SCOTT |
| KING - JONES - FORD |
| KING - BLAKE - ALLEN |
| KING - BLAKE - WARD |
| KING - BLAKE - MARTIN |
| KING - BLAKE - TURNER |
| KING - BLAKE - JAMES |
| KING - CLARK - MILLER |
| KING - JONES - SCOTT - ADAMS |
| KING - JONES - FORD - SMITH |
+------------------------------+
14 rows in set (0.00 sec)
