Question

I have a database of assessment outcomes, with fields identifying the pupil, grade, the year and term. The query below finds me pupils who have an assessment for a specific subject at some start and end point:

SELECT
  a1.PupilId
FROM assessment as a1
  JOIN assessment as a2
    USING (PupilId)
WHERE a1.GradeID in('1A','1B','1C')
    AND a2.GradeID in('5A','5B','5C')
    AND a1.NCYear = '1'
    AND a1.Term = '6'
    AND a2.NCYear = '2'
    AND a2.Term = '6'
    AND a1.Subject = '31'
    AND a2.Subject = '31'
    AND a1.Type = 'assessment'
    AND a2.Type = 'assessment'

This works fine, but now I want to find pupils who have the starting grade (i.e. NCYear=’1′ and Term=’6′) but don’t have an entry for the end grade. I’ve played around with NOT EXISTS, but can’t get it to work.

To clarify, here’s the assessment table:

id | Subject | PupilId | NCYear | Term | GradeID | Type
--------------------------------------------------------
1  | 31      | 1       | 1      | 6    | 1A      | assessment
2  | 31      | 1       | 2      | 6    | 5A      | assessment
3  | 31      | 2       | 1      | 6    | 1A      | assessment

The SQL query above should return PupilID 1. I need the query which will return PupilID 2, the start assessment is there but no end one.

Any suggestions?

Answer 1

You need to use an OUTER JOIN:

SELECT
  a1.PupilId
FROM assessment as a1
  OUTER JOIN assessment as a2
    ON a2.PupilId = a2.PupilId AND a2.NCYear = '2' AND a2.GradeID in('5A','5B','5C') AND a2.Term = '6' AND a2.Subject = '31' AND a2.Type = 'assessment'        
WHERE a1.GradeID in('1A','1B','1C')
    AND a1.NCYear = '1'
    AND a1.Term = '6'
    AND a1.Subject = '31'
    AND a1.Type = 'assessment'
    AND a2.PupilId IS NULL