SQL - Mixing ANDs and ORs

Question

Hello,

I'm not a SQL Expert. I'm using SQL Server 2005 and I'm trying to figure out how to structure a query so that it can meet several requirements. I have two tables defined as follows:

Classroom
 - ID
 - DepartmentID
 - TeacherName
 - RoomNumber

Student
 - ID
 - Name
 - ClassroomID

I'm trying to build a query that says, "give me the classrooms in department [x] OR department [y] that have more than 30 students AND give me the classrooms in department [w] OR department [z] that have more than 40 students. I'm confused on how to mix the ANDs and the ORs properly in my SQL. Currently, I am trying the following:

SELECT
  c.RoomNumber,
  c.TeacherName
FROM
  Classroom c
WHERE
  c.[DepartmentID]=5 AND (SELECT COUNT(*) FROM Student s WHERE s.ClassroomID=c.ID > 30) OR
  c.[DepartmentID]=6 AND (SELECT COUNT(*) FROM Student s WHERE s.ClassroomID=c.ID > 30) OR
  c.[DepartmentID]=7 AND (SELECT COUNT(*) FROM Student s WHERE s.ClassroomID=c.ID > 40) OR
  c.[DepartmentID]=8 AND (SELECT COUNT(*) FROM Student s WHERE s.ClassroomID=c.ID > 40)

What am I doing wrong? Thank you!

Answer 1

Try this

(SELECT COUNT(*) FROM Student s WHERE s.ClassroomID=c.ID) >30

instead of this

(SELECT COUNT(*) FROM Student s WHERE s.ClassroomID=c.ID > 30)

;]

Answer 2

You need to add a set of parens around the first two boolean expressions that are joined by the AND

should be

(...) OR
(...) 

etc

Answer 3

Be very explicit with your parens.

Notice that I added 3 more sets of parens.

((c.[DepartmentID]=5) AND ((SELECT COUNT(*) FROM Student s WHERE s.ClassroomID=c.ID) > 30)) OR

Answer 4

SELECT  c.id
FROM    Classroom c
JOIN    Students s
ON      s.Classroom = c.Id
WHERE   DepartmentID IN ('X', 'Y', 'W', 'Z')
GROUP BY
        c.id, c.DepartmentID
HAVING  COUNT(*) >= CASE WHEN DepartmentID IN ('X', 'Y') THEN 30 ELSE 40 END