一、with as 公用表表達(dá)式
類似VIEW，但是不并沒有創(chuàng)建對(duì)象，WITH AS 公用表表達(dá)式不創(chuàng)建對(duì)象，只能被后隨的SELECT語句，其作用：
1. 實(shí)現(xiàn)遞歸查詢(樹形結(jié)構(gòu))
2. 可以在一個(gè)語句中多次引用公用表表達(dá)式，使其更加簡潔
二、非遞歸的公共表達(dá)式
可以是定義列或自動(dòng)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
--自動(dòng)列
with withTmp2
as
(
select * from ClassUnis
where Author = 'system'
)
select * from withTmp2
三、遞歸的方式
通過UNION ALL 連接部分。通過連接自身whit as 創(chuàng)建的表達(dá)式，它的連接條件就是遞歸的條件?？梢詮母?jié)點(diǎn)往下查找，從子節(jié)點(diǎn)往父節(jié)點(diǎn)查找。只需要顛倒一下連接條件。例如代碼中條件改為t.ID = c.ParentId即可
with tree as(
--0 as Level 定義樹的層級(jí),從0開始
select *,0 as Level
from ClassUnis
where ParentId is null
union all
--t.Level + 1每遞歸一次層級(jí)遞增
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'%/%'
還能通過option(maxrecursion Number) 設(shè)置最大遞歸次數(shù)。例如上訴結(jié)果Level 最大值為2表示遞歸兩次。我們?cè)O(shè)置其值為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)
好了這篇文章就介紹到這了，希望能幫助到你。
