SQL學習筆記6 with as的用法

一、with as 短語含義

with as短語，也叫做子查詢部分（subquery factoring），可以定義乙個sql片斷，該sql片斷會被整個sql語句用到。可以使sql語句的可讀性更高，也可以在union all的不同部分，作為提供資料的部分。減少巢狀查詢可以提公升sql的查詢效率

二、with as 用法

例如之前的join例項：

select stu_name,grade from

student
join
(select stu_id,grade from course 
join 
grade
on grade.stu_id=course.stu_id
where couname='語文') as a
on a.stu_id=student.stu_id

with t1 

as select stu_id
,grade 
from cource 
join 
grade 
on grade.stu_id=course.stu_id
where couname='語文'
select stu_name
,grade
from t1 
join 
student
on student.stu_id=t1._stuid

with t1 

as select id from table_a
,t2 as select id from table_b
,t3 as select id from table_c
selet
t1.id from 
t1join 
t2on t1.id=t2.id
join
t3on t2.id=t3.id

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