Aggregate function in SQL Server

Question

I'm getting really frustrated about SQL Server. I'm just trying to join 3 tables, very simple and easily done in mysql. But in SQL Server it keeps telling me to contain tbl_department.deptname in an aggregate function. But what aggregate function could I possibly use in a simple string?

SELECT      
    COUNT(tblStudent_Department.student_id) AS Expr2, 
    tbl_department.deptname AS Expr1
FROM          
    tblStudent_Department 
LEFT OUTER JOIN
    tbl_department ON tblStudent_Department.deptcode = tbl_department.deptcode 
LEFT OUTER JOIN
    tblStudent ON tblStudent_Department.student_id = tblStudent.studentid
GROUP BY 
    tblStudent_Department.deptcode

Please help.

Answer 1

The database doesn't know that if you group on deptcode , you're implicitly grouping on deptname . You must tell SQL Server this by adding the column to the group by :

GROUP BY tblStudent_Department.deptcode, tbl_department.deptname

MySQL is special in that it basically picks a random row if you don't specify an aggregate. This can be misleading and lead to wrong results. As in many other things, MySQL has the more pragmatic solution, and SQL Server the more correct one.

Answer 2

The problem is because your GROUP BY and SELECT terms don't match up.

The simplest way to fix this is to add tbl_department.deptname into your GROUP BY , like so:

GROUP BY tblStudent_Department.deptcode, tbl_department.deptname

Answer 3

You're grouping by deptcode but selecting deptname - if you don't want to aggregate the department (which sounds like it makes sense) then you need to have the deptname in the "group by" statement:

SELECT COUNT(tblStudent_Department.student_id) AS Expr2, tbl_department.deptname AS Expr1
FROM tblStudent_Department 
  LEFT OUTER JOIN tbl_department ON tblStudent_Department.deptcode = tbl_department.deptcode 
  LEFT OUTER JOIN tblStudent ON tblStudent_Department.student_id = tblStudent.studentid
GROUP BY tblStudent_Department.deptname 

Note I've removed the deptcode because I don't think you need it

If you're using aggregate functions (sum, count etc) ALL fields returned in your select statement need to either be aggregated OR in the group by clause.

Answer 4

First, Last, or put it into the group by.

The rule are:

• IF you use a group by, every field is either one of the grouping fields OR one of the aggregated fields.

If you select tbl_department.deptname then you have to either group by that, too, or say WHICH ONE is taken.

Some aggretgate functions are faking that nicely - First, Last (take first or last occurance).