SQL和計數

Question

我有包括以下

Student Name  Grade  Class 
-------------------------------------------------------- 
User 1    A   English 
User 1    B   Math 
User 2    B   Math 

我試圖創建一個查詢，它會列出所有學生的表格，總通等級（等級A，B & C） ，總數A，總數B，總數C

有人可以幫助我創建查詢或至少讓我在正確的方向嗎？

Answer 1

試試這個：

select name, 
    count(case when grade in ('A', 'B', 'C') then 1 end) totalPass, 
    count(case when grade = 'A' then 1 end) totalA, 
    count(case when grade = 'B' then 1 end) totalB, 
    count(case when grade = 'C' then 1 end) totalC 
from t 
group by name 

這裏是fiddle。

或者我們也可以，如果你使用MySQL使它更簡單：

select name, 
    sum(grade in ('A', 'B', 'C')) totalPass, 
    sum(grade = 'A') totalA, 
    sum(grade = 'B') totalB, 
    sum(grade = 'C') totalC 
from t 
group by name 

這裏是fiddle。

Answer 2

您應該使用sum代替count，並使用內部條件表達式，如：

select 
    sum(case when grade in ('A','B','C') then 1 else 0 end) as passing 
, sum(case when grade = 'A' then 1 else 0 end) as NumA 
, sum(case when grade = 'B' then 1 else 0 end) as NumB 
, sum(case when grade = 'C' then 1 else 0 end) as NumC 
from grades 

這個例子是SQL Server (see sqlfiddle here)，但其他RDBMS具有類似的功能。

Answer 3

另一種可能的答案（很長，但是自我解釋）：

select u.UserName 
,totalA = count(a.Grade) 
,totalB = count(b.Grade) 
,totalC = count(c.Grade) 
,totalPass = count(p.Grade) 
from 
GradeUser u 
LEFT JOIN GradeUser a on a.UserName = u.UserName 
AND a.Grade = u.Grade AND a.Class = u.Class 
AND a.Grade ='A' 
LEFT JOIN GradeUser b on b.UserName = u.UserName 
AND b.Grade = u.Grade AND b.Class = u.Class 
AND b.Grade ='B' 
LEFT JOIN GradeUser c on c.UserName = u.UserName 
AND c.Grade = u.Grade AND c.Class = u.Class 
AND c.Grade ='C' 
LEFT JOIN GradeUser p on p.UserName = u.UserName 
AND p.Grade = u.Grade AND p.Class = u.Class 
AND p.Grade IN ('A', 'B', 'C') 
GROUP BY u.UserName 

Answer 4

這裏是訪問SQL的解決方案，在兩個單獨的查詢：

每名學生的總合格成績的數目：

SELECT grades.StudentName, Count(grades.Grade) AS NumberOfPassGrades 
FROM grades 
WHERE (((grades.Grade) In ('A','B','C'))) 
GROUP BY grades.StudentName 
ORDER BY grades.StudentName; 

的總A的號碼，共有B的，總的C的：

SELECT grades.StudentName, grades.Grade, Count(grades.Grade) AS NumberOfGrades 
FROM grades 
GROUP BY grades.StudentName, grades.Grade 
ORDER BY grades.StudentName, grades.Grade; 

如果您確實只需要一個查詢，最簡單的方法是將這兩個查詢保存爲Access查詢並將它們加入StudentName。然而，一旦學生有不止一個不同的成績，你就會多次獲得NumberOfPassGrades。