为什么此查询不起作用

Question

There are two tables, both have only one column. delete_DimCSI has 196194 rows and delete_FRO has 195740 rows. I am looking for rows that exist in delete_DimCSI table but not existing in delete_FRO table.

This is the query I usually use and it worked so far:

select PK_CSI from delete_DimCSI
where PK_CSI not in (select FK_CSI from delete_FRO)

It returns 0 rows.

Then I created this one:

select PK_CSI, FK_CSI from delete_DimCSI
LEFT OUTER JOIN delete_FRO FRO on FK_CSI = PK_CSI
where FK_CSI is null

It returns 455 rows.

Any idea why the first query doesn't return any row?

Answer 1

You probably have NULL s in FK_csi

select PK_CSI from delete_DimCSI
where PK_CSI not in (select FK_CSI from delete_FRO WHERE FK_CSI IS NOT NULL)

Look here for an explanation.

Answer 2

NOT IN behaves strangely when the subquery has NULL values. In that case, it returns either NULL or false -- neither of which is treated as true.

For this reason, I recommend using NOT EXISTS rather than NOT IN . The semantics are more what you expect:

select PK_CSI
from delete_DimCSI d
where not exists (select 1 from delete_FRO f where f.FK_CSI = d.PK_CSI);

Of course, you can also add an explicit where f.FK_CSI = NULL` as Mihai suggests.