Re: Joins with nulls

In message <6278687.0211141337.3ac55b35_at_posting.google.com>, Juan Pardillos <sicotom_at_eresmas.com> writes
>Hi,
>
>I've read that performing joins with tables that have null values in
>some attributes is a dangerous operation, but no example was given. I
>think that this only can happen if you try to join based on the
>attributes that can have two null values, due to the problem of how to
>interpret null values (are two null values equal?). Can this also
>happen in other circumstances?.

It's not that joins with nulls are difficult, joins with nulls are impossible. When you join two records you usually take a record from each table where the join fields have the same value in each record. That's not possible when either of the fields have a null value. You cannot say that the null equals any value, and therefore cannot say that one null is equal to another null value.

-- 
Bernard Peek
bap_at_shrdlu.com
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