Question

I did previously ask a question about this, but must admit was biased against SQL based solutions and now I can’t find the question, so I’ll try again, phrase it carefully and be much more open to any solutions provided.

I have two tables:

tblCurrent 
 Ref  | CustomerID | ... .. .. .  . |
X001
X002
X003


tblHistorical 
 Ref   | ... .. .. .  . |  Missing | Matched
X001   | ... .. .. .  . |  TRUE    | FALSE
X001   | ... .. .. .  . |  FALSE   | FALSE
X002   | ... .. .. .  . |  TRUE    | TRUE
X002   | ... .. .. .  . |  TRUE    | FALSE
X003   | ... .. .. .  . |  FALSE   | FALSE
X003   | ... .. .. .  . |  TRUE    | TRUE

Ref is unique in tblCurrent, but not in Historical.

How can I construct a view, that is based on tblCurrent that results in three additional columns that count the number of records in tblHistorical that:

• match the Ref AND Missing is TRUE

• match the Ref AND Missing is False

• match the Ref AND Matched is True

Note that I will need to add additional columns that count the number of records in tblHistorical based on similar criteria.

Answer 1

I think you need to use LEFT JOIN on this since it is possible that sometime ref may not exist on tblHistorical table.

SELECT  a.ref,
        SUM(CASE WHEN b.Missing = 'True' THEN 1 ELSE 0 END) missingTrue,
        SUM(CASE WHEN b.Missing = 'False' THEN 1 ELSE 0 END) missingFalse,
        SUM(CASE WHEN b.Matched = 'True' THEN 1 ELSE 0 END) matchTrue
FROM    tblCurrent a
        LEFT JOIN tblHistorical b
            on a.ref = b.ref
GROUP BY a.ref