Why does my left join in Access have fewer rows than the left table?

Question

I have two tables in an MS Access 2010 database: TBLIndividuals and TblIndividualsUpdates. They have a lot of the same data, but the primary key may not be the same for a gi

Answer 1

This should never happen. Unless rows are being inserted/deleted in the meantime,

the query:

SELECT *
FROM a LEFT JOIN b
         ON whatever ;

should never return less rows than:

SELECT *
FROM a ;

If it happens, it's a bug. Are you sure the queries are exactly like this (and you have't omitted some detail, like a WHERE clause)? Are you sure that the first returns 4149 rows and the second one 4103 rows? You could make another check by changing the * above to COUNT(*).

Answer 2

ON ( 

        TblIndividuals.FirstName = TblIndividualsUpdates.FirstName

        and 

        TblIndividuals.LastName = TblIndividualsUpdates.LastName

        AND (
                 TblIndividuals.DateBorn = TblIndividualsUpdates.DateBorn      
                 or 
                 (
                     TblIndividuals.DateBorn is null          
                     and 
                     (
                     TblIndividuals.MidName is null 
                     and TblIndividualsUpdates.MidName is null              
                     or TblIndividuals.MidName = TblIndividualsUpdates.MidName
                     )
                 )
             )
    );

What I would do is systematically remove all the join conditions except the first two until you find the records drop off. Then you will know where your problem is.

Answer 3

Drop any indexes from both tables which include those JOIN fields (FirstName, LastName, and DateBorn). Then see whether you get the expected 4,149 rows with this simplified query.

SELECT
    i.PersonID AS OldID, 
    u.PersonID AS UpdateID
FROM
    TblIndividualsUpdates AS u
    LEFT JOIN TblIndividuals AS i
    ON
        (
            (i.FirstName = u.FirstName) 
        AND (i.LastName = u.LastName) 
        AND (i.DateBorn = u.DateBorn)
        );

Answer 4

For whatever it is worth, since this seems to be a deceitful bug and any additional information could help resolving it, I have had the same problem.

The query is too big to post here and I don't have the time to reduce it now to something suitable, but I can report what I found. In the below, all joins are left joins.

I was gradually refining and changing my query. It had a derived table in it (D). And the whole thing was made into a derived table (T) and then joined to a last table (L). In any case, at one point in its development, no field in T that originated in D participated in the join to L. It was then the problem occurred, the total number of rows mysteriously became less than the main table, which should be impossible. As soon as I again let a field from D participate (via T) in the join to L, the number increased to normal again.

It was as if the join condition to D was moved to a WHERE clause when no field in it was participating (via T) in the join to L. But I don't really know what the explanation is.