DBA SQL开发知识：SQL Server 通过with as方法查询树型结构
一、with as 公用表表达式
类似VIEW，但是不并没有创建对象，WITH AS 公用表表达式不创建对象，只能被后随的SELECT语句，其作用：
1. 实现递归查询(树形结构)
2. 可以在一个语句中多次引用公用表表达式，使其更加简洁
二、非递归的公共表达式
可以是定义列或自动列和select into 效果差不多
–指定列
with withTmp1 (code,cName)
as
(
select id,Name from ClassUnis
)
select * from withTmp1
–自动列
with withTmp2
as
(
select * from ClassUnis
where Author = ‘system’
)
select * from withTmp2
三、递归的方式
通过UNION ALL 连接部分。通过连接自身whit as 创建的表达式，它的连接条件就是递归的条件。可以从根节点往下查找，从子节点往父节点查找。只需要颠倒一下连接条件。例如代码中条件改为t.ID = c.ParentId即可
with tree as(
–0 as Level 定义树的层级,从0开始
select *,0 as Level
from ClassUnis
where ParentId is null
union all
–t.Level + 1每递归一次层级递增
select c.*,t.Level + 1
from ClassUnis c,tree t
where c.ParentId = t.ID
–from ClassUnis c inner join tree t on c.ParentId = t.ID
)
select * from tree where Author not like’%/%’
还能通过option(maxrecursion Number) 设置最大递归次数。例如上诉结果Level 最大值为2表示递归两次。我们设置其值为1
with tree as(
select *,0 as Level from ClassUnis where ParentId is null
union all
select c.*,t.Level + 1 from ClassUnis c,tree t where c.ParentId = t.ID
)
select * from tree where Author not like’%/%’
option(maxrecursion 1)
好了这篇文章就介绍到这了，希望能帮助到你。
