/*
子查詢(乙個查詢的結果作為另外乙個查詢的條件
一般子查詢要保證返回結果只有乙個(即一行,一列)in子查詢返回一列多行
子查詢從內往外讀(寫)
當要顯示多個表資料時使用聯結，只顯示乙個表資料時使用子查詢)*/
select * from student where
stuage > (select stuage from student where stuname = '李四') --
select
* from
student stu
inner join
exam exa
on(stu.stuid = exa.stuid)
where
exa.writtenaxam > 95
--select
stu.*, exa.*
from
student as stu, exam exa
where
stu.stuid in (select stuid from exam where exa.writtenaxam > 95)
andstu.stuid = exa.stuid
--select
*from
student
where
stuid in (select stuid from exam where writtenaxam > 95)
--select
*from
student
where
stuid not in (select stuid from exam where writtenaxam > 95)
--if exists(select * from student)
print '有資料'
else
print '無資料'
--if exists(select * from student where stuid = '199')
print '有資料'
else
print '無資料'
--取反not exists
if not exists(select * from student where stuid = '199')
print '有資料'
else
print '無資料'
--第一種寫法
if exists(select * from exam where writtenaxam >= 80)
update exam set writtenaxam = writtenaxam + 2
else
update exam set writtenaxam = writtenaxam + 5
--第二種寫法
declare @score int
set @score = 5
if exists(select * from exam where writtenaxam >= 80)
set @score = 2
update exam set writtenaxam = writtenaxam + @score
--取常量列的2種方式如下
select *, '是否通過' = '是' from student
select *, 是否通過 = '是' from student
select *, '是' as '是否通過' from student
select *, '是' as 是否通過 from student
select *, '是' '是否通過' from student --as關鍵字也可以省略
--select
* ,'等級' =
case
when labexam >= 90 then '優秀'
when labexam >= 80 and labexam <= 90 then '及格'
else '不及格'
endfrom exam
--select *, '計算結果' = 2 / 5 from student
select *, '計算結果' = (2 * 1.0) / (5 * 1.0) from student
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