select id from classes where name ='一期'
select* from students where cid =(select id from classes where name ='一期')--子查詢
select * from students where cid in (select id from classes where name ='一期'
select * from students where cid not in (select id from classes where name ='一期')
select * from students where exists (select * from classes where classes.id=students .cid) --主查詢會根絕子查詢的條件進行匹配
select * from students where not exists (select * from classes where classes.id=students .cid)
--第一種分頁方式，如果要取很多頁之後的資料效能比較低
select top 10 * from students where id in (select top 10 id from students )--查出前十條資料
select top 10 * from students where id not in (select top 10 id from students )--查出第二頁資料
--第二種分頁比較高效，row_number()方法會給結果集編號
select * from (
select *,row_number()over(order by id) as rowsnumber from students ) as t
where t.rowsnumber between 1 and 10
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