Question

It’s possible that this question has already been asked, but I can’t seem to find my answer, so here goes:

I’m selecting 10 fields from a JOIN on two tables. The result is about 1186 rows of data. In these rows, several are duplicates for all intents and purposes, except for one unique field (ClassId). I need this field, but it’s keeping me from getting and what I have defined as “Unique”. For example, querying on only Code, TeacherDescrip, and Term yields 1120 records.

I was hoping to run this same query, but add a WHERE clause with a subquery that searched for a narrower set of fields, the three listed above to be exact, that excluded that unique field that was causing my “duplicates”. Of course, the error I received is below:

“Only one expression can be specified in the select list when the
subquery is not introduced with EXISTS.”

When i tried to use EXISTS in stead of ClassScedule.Code IN I still got back the full 1186.

here is my query:

 SELECT DISTINCT ClassId
      ,Code
      ,Section
      ,Course
      ,Students
      ,ClassStart
      ,TeacherDescrip
      ,AdTeacherID
      ,email
      ,Term
      ,Campus
  FROM ClassScedule
  JOIN staff ON staff.StaffID = ClassScedule.AdTeacherID
  WHERE ClassStart BETWEEN '2012-03-01' AND '2012-03-30'
        AND ClassScedule.Code IN 
              (SELECT DISTINCT ClassScedule.Code, TeacherDescrip, Termcode 
               FROM ClassScedule 
              WHERE ClassStart BETWEEN '2012-03-01' AND '2012-03-30')
        AND TeacherDescrip IS NOT NULL 
  ORDER BY Instructor

Answer 1

If you are using SQL Server 2005 or later, you can select one result for each distinct (Code, TeacherDescrip, Term) as follows:

SELECT TOP (1) WITH TIES
       ClassId
      ,Code
      ,Section
      ,Course
      ,Students
      ,ClassStart
      ,TeacherDescrip
      ,AdTeacherID
      ,email
      ,Term
      ,Campus
  FROM ClassScedule
  JOIN staff ON staff.StaffID = ClassScedule.AdTeacherID
  WHERE ClassStart BETWEEN '2012-03-01' AND '2012-03-30'
  AND TeacherDescrip IS NOT NULL 
  ORDER BY ROW_NUMBER() OVER (
    PARTITION BY Code, TeacherDescrip, Term
    ORDER BY ClassID DESC
  )

Among duplicate (Code, TeacherDescrip, Term) values, this query gives you the result with the smallest ClassID. If you want the largest instead, remove DESC.