Re: What's wrong with this query?

Albe Laurenz, 22.06.2009 09:52:
> Sorry to be nitpicking, but maybe in that case it adds to clarity:
>
> A comparison with NULL does not return FALSE, but "undefined" or NULL.
>
> Try to run the following queries:
>
> SELECT 1 = 2;
> and
> SELECT 1 = NULL;
>
> and observe the different result.
>
> In the context of the original question this difference does not matter,
> because a comparison is considered successful only if it returns TRUE.
>
> But I think this way it becomes clearer *why* neither = nor != will
> succeed for a NULL (= undefined) value: if you don't know which value
> a certain thing has, you can neither say that it is equal to 1 nor
> that it is not equal to 1.

Good points :)

Thanks for the clarification!

I recently saw a blog talking about interview questions. One of them was:


Under which circumstances does the following query *not* return all rows

SELECT *
FROM the_table
WHERE some_column = some_column;

boils down to the same behaviour...

Regards
Thomas