Question

Let’s say we have a table (EnsembleMembers) in an SQL database with the following data. It lists the musicians which are part of various ensembles, along with their instruments.

EnsembleID (FK)   MusicianID (FK)   Instrument
----------------------------------------------
'1'               '1'               'Clarinet'
'1'               '4'               'Clarinet'
'1'               '100'             'Saxophone'
'2'               '200'             'Saxophone'
'2'               '300'             'Saxophone'
'2'               '320'             'Flute'
'99'              '300'             'Clarinet'

I want to select the ensemble IDs where the ensemble has one or more saxophone or one or more clarinet players, but not both. I have tried the following SQL statement, but it is returning 1,2,2,99, rather than the expected 2,99.

SELECT e1.EnsembleID 
  FROM ensemblemembers e1 
 WHERE e1.Instrument = 'Saxophone' 
    OR e1.Instrument = 'Clarinet' 
  AND NOT EXISTS (SELECT * 
                    FROM ensemblemembers e2 
                   WHERE (    e1.Instrument = 'Saxophone' 
                          AND e2.Instrument = 'Clarinet' 
                          AND e1.EnsembleID = e2.EnsembleID) 
                      OR (    e1.Instrument = 'Clarinet' 
                          AND e2.Instrument = 'Saxophone' 
                          AND e1.EnsembleID = e2.EnsembleID));

What am I doing wrong?

PS – I don’t want to use DISTINCT for performance reasons.

Answer 1

I assume you have a table called ENSEMBLES:

select E.id from ensembles E where exists (
  select 1 from ensemblemembers M where E.id = M.ensembleid
  and M.instrument in ('clarinet', 'saxophone')
) and not (
  exists (
    select 1 from ensemblemembers M where E.id = M.ensembleid
    and M.instrument = 'clarinet'
  ) and exists (
    select 1 from ensemblemembers M where E.id = M.ensembleid
    and M.instrument = 'saxophone'
  )
)

You want to avoid using DISTINCT, so one way to do it is by using the main ENSEMBLES table. From there, pick ensemble rows that have ‘clarinet’ OR ‘saxophone’. Then the third step is to remove all ensemble rows that have ‘clarinet’ AND ‘saxophone’.