“<>” vs “NOT IN”

Question

I was debugging a stored procedure the other day and found some logic something like this:

SELECT something
FROM someTable
WHERE idcode <> (SELECT ids          


        

Answer 1

in some versions of SQL != should be used for a "not equals" logical statement. Have you tried that?

Answer 2

SELECT something
FROM someTable
WHERE idcode NOT IN (SELECT ids FROM tmpIdTable)

checks against any value in the list.

However, the NOT IN is not NULL-tolerant. If the sub-query returned a set of values that contained NULL, no records would be returned at all. (This is because internally the NOT IN is optimized to idcode <> 'foo' AND idcode <> 'bar' AND idcode <> NULL etc., which will always fail because any comparison to NULL yields UNKNOWN, preventing the whole expression from ever becoming TRUE.)

A nicer, NULL-tolerant variant would be this:

SELECT something
FROM someTable
WHERE NOT EXISTS (SELECT ids FROM tmpIdTable WHERE ids = someTable.idcode)

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EDIT: I initially assumed that this:

SELECT something
FROM someTable
WHERE idcode <> (SELECT ids FROM tmpIdTable)

would check against the first value only. It turns out that this assumption is wrong at least for SQL Server, where it actually triggers his error:

Msg 512, Level 16, State 1, Line 1
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

Answer 3

<> is a "singular" NOT operation; NOT IN is a set operation, so it makes sense that the former wouldn't work. I have no idea whether or not it may have done so under a previous version of SQL Server, however.