select a.classname,a.coursename,sum(不及格) as 不及格,sum(差) as 差,sum(中等) as 中等,sum(好) as 好 ,sum(不及格)+sum(差)+sum(中等)+sum(好) as 班級總人數 from (select studentid,classname,coursename,1 as 不及格,0 as 差,0 as 中等,0 as 好 from studentscore where scoreremark='fail' union all
select studentid,classname,coursename,0 as 不及格,1 as 差,0 as 中等,0 as 好 from studentscore where scoreremark='low' union all
select studentid,classname,coursename,0 as 不及格,0 as 差,1 as 中等,0 as 好 from studentscore where scoreremark='medium' union all
select studentid,classname,coursename,0 as 不及格,0 as 差,0 as 中等,1 as 好 from studentscore where scoreremark='good' )a group by classname,coursename
在這裡需要注意的是要給某些欄位加別名以示區別！
一張表中, 查詢學生及格和不及格人數
表結構為 class name score 班級 姓名 分數 查詢每個班級中,及格和不及格學生的人數 select class sum case when score 60 then 1 else 0 end as gt60,sum case when score 60 then 0 else 1 ...
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1.選擇運算應盡量先做。最重要，最基本的一條。2.執行連線前應對關係適當的預處理。student 與 sc 進行自然連線 a.索引連線 a.1.在表sc上建立sno索引。a.2 對student組中的每乙個元組，有sno值通過sc的索引查詢相應的sc元組 a.3 把student和sc連線起來 b....