递归分两种：一种由父项向下级递归，另一种是由子项向上级递归。下面就这两种情况做个简单的处理。

假设有一个表treeview，包含字段 id,parentid,text 分别代表id，上级id，描述字段（这里就不把建表sql写出来了）。

一、Sqlserver中的写法

1、由父项递归下级

with cte(id,parentid,text)
as
(--父项
select id,parentid,text from treeview where parentid = 450 --需替换成自己希望查询的id
union all
--递归结果集中的下级
select t.id,t.parentid,t.text from treeview as t
inner join cte as c on t.parentid = c.id
)
select id,parentid,text from cte

2、由子级递归父项

with cte(id,parentid,text)
as
(--下级父项
select id,parentid,text from treeview where id = 450 --需替换成自己希望查询的id
union all
--递归结果集中的父项
select t.id,t.parentid,t.text from treeview as t
inner join cte as c on t.id = c.parentid
)
select id,parentid,text from cte

二、Oracle中的写法

　1、由父项递归下级

select a.id,a.parentid,a.text
from treeview a
start with a.id=450 --需替换成自己希望查询的id
connect by prior a.id = a.parentid

　2、由子级递归父项

select a.id,a.parentid,a.text
from treeview a
start with a.id=450 --需替换成自己希望查询的id
connect by prior a.parentid = a.id

三、PostgreSQL中的写法

1、由父项递归下级

with RECURSIVE cte (id,parentid,text)
as
(
select id,parentid,text from treeview where id = 10 --需替换成自己希望查询的id
union all
select t.id,t.parentid,t.text from treeview t,cte t1 where t1.id=t.parentid
)
select * from cte order by rpad(id::varchar,5,'0') asc;

　2、由子级递归父项

with RECURSIVE cte (id,parentid,text)
as
(
select id,parentid,text from treeview where id = 111 --需替换成自己希望查询的id
union all
select t.id,t.parentid,t.text from treeview t,cte t1 where t1.parentid=t.id
)
select * from cte order by rpad(id::varchar,5,'0') asc;

四、Mysql中的写法

　　Mysql的写法很多，网上许多使用自建函数的，这里介绍一种更为通用的方法。

1、由父项递归下级

SELECT t3.*
FROM
(
SELECT t1.id,t1.parentid,t1.text,
IF ( find_in_set( t1.parentid, @pids ) > 0, @pids := concat( @pids, ',', t1.id ), 0 ) AS ischild
FROM
( SELECT id,text,parentid FROM treeview t WHERE 1 = 1 ORDER BY parentid, id ) t1,
( SELECT @pids := 450 ) t2 --替换成自己想要查询的id
) t3
WHERE t3.ischild != 0

　2、由子级递归父项

SELECT id,text,parentid,t1.lvl
FROM (
SELECT
@r AS _id,
(SELECT @r := parentid FROM treeview WHERE id = _id) AS parent_id,
@l := @l + 1 AS lvl
FROM
(SELECT @r := 450, @l := 0) vars, --替换成自己想要查询的id
treeview h
WHERE @r <> 0
) T1
JOIN treeview T2 ON T1._id = T2.id
ORDER BY id;

简单的例子，供大家学习参考。