SQL and Counting | 易学教程

问题

I have a table which consists of the following

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

I'm trying to create a query where it will list all students, total pass grades (grades A,B & C), total A's, Total B's, Total C's

Can someone help me create the query or atleast put me in the right direction?

回答1:

Give this a try:

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

Here is the fiddle.

Or we can make it even simpler if you were using MySQL:

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

Here is the fiddle.

回答2:

You should use sum instead of count, and use a conditional expression inside, like this:

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

This example is for SQL Server (see sqlfiddle here), but other RDBMSs have similar capabilities.

回答3:

Yet another possible answer (very long but self explanatory):

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

回答4:

Here is a solution in Access SQL, in two separate queries:

Number of total pass grades per student:

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;

Number of total A's, Total B's, Total C's:

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

If you really need only one query, the easiest way to do this would be to save both queries as Access queries and join them on StudentName. However, then you would get the NumberOfPassGrades more than once as soon as a student has more than one different grade.

来源：https://stackoverflow.com/questions/10240035/sql-and-counting

标签

sql

ms-access

aggregate-functions