优化SQL查询与计数表的多个连接

Question

无论如何，如果没有我向您显示表格，您将能够帮助优化此查询？所有这些疑问都源于

我原来的表有以下几列和表名为laterec-学生

-------------------------------------------------------------- 
| studentid | name | class | latetime    | waived | 
-------------------------------------------------------------- 
| ID1111STU | Stu 1 | 1A |2012-01-09 08:09:00 |Waived | 



SELECT A.class, NoStudentsLate, 1xLATE, 2xLATE FROM (

    SELECT 
     class, 
     count(DISTINCT studentid) AS NoStudentsLate 
    FROM `laterec-students` 
    WHERE waived!="Waived" 
    GROUP BY class 

) AS A 
LEFT JOIN (

    SELECT class, count(distinct studentid) AS 1xLATE from (
     SELECT `laterec-students`.class, `laterec-students`.studentid 
     FROM `laterec-students` 
     WHERE waived!="Waived" 
     GROUP BY studentid 
     HAVING count(studentid)=1) as temp 
    GROUP BY class 
) AS B ON A.class=B.class 

LEFT JOIN (
    SELECT class, count(distinct studentid) AS 2xLATE from (
    SELECT `laterec-students`.class, `laterec-students`.studentid 
     FROM `laterec-students` 
    WHERE waived!="Waived" 
    GROUP BY studentid 
    HAVING count(studentid)=2) as temp 
    GROUP BY class 
) AS C ON A.class=C.class 

这就是我试图完成

--------------------------------------------------------------------- 
| Class | Total # of students late | # late 1 times | # late 2 times | 
--------------------------------------------------------------------- 
| 1A | 5      |  3   |  2   | 
| 1B | 3      |  3   |  0   | 
--------------------------------------------------------------------- 

所以这是什么意思，在1A班，总共有5名学生迟到使用学生ID计算。在这5人中，3人迟到一次，2人迟到两次。

再次在1B班，共有3名学生迟到了，他们都只迟了一次。

Answer 1

我希望我了解您的查询，但以下内容适用于我的SQL Fiddle example。

SELECT 
    class, 
    SUM(cnt > 0) AS NoStudentsLate, 
    SUM(cnt = 1) AS 1xLate, 
    SUM(cnt = 2) AS 2xLate 
FROM 
(
    SELECT class, studentid, COUNT(*) AS cnt 
    FROM `laterec-students` 
    WHERE waived!='Waived' 
    GROUP BY class, studentid 
) t 
GROUP BY class;