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

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.