Q:

Optimize SQL query multiple joins with count tables

By any chance you would be able to help optimize this query without me showing you the tables?
My original table that all of these queries are derived from has the following columns and the table is named laterec-students
--------------------------------------------------------------
| 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

This is what I am trying to accomplish
---------------------------------------------------------------------
| Class | Total # of students late | # late 1 times | # late 2 times |
---------------------------------------------------------------------
| 1A    |    5                     |     3          |     2          |
| 1B    |    3                     |     3          |     0          |
---------------------------------------------------------------------

So what this means, in class 1A, there are a total of 5 student late as counted using the student id. Out  of this 5, 3 students are late once, and 2 students are late twice.
Again in class 1B, total 3 students are late, and all of them are only late once.

A:

I hope that I understood your query, but the following works with my 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;

