Using nullable columns in a WHERE clause

Question

Assume a table definition in SQL Server as follows:

CREATE TABLE MyTable (
  Id   UNIQUEIDENTIFIER NULL,
  Info VARCHAR(MAX)
)

And a query:

DECLARE @id UNIQUEIDENTIFIER
DECLARE @info VARCHAR(MAX)
IF @id IS NOT NULL
BEGIN
  SELECT @info = Info
    FROM MyTable
    WHERE Id = @id
END

In that case, the Visual Studio static code analyzer produces the following error:

Warning : SR0007 : Microsoft.Performance : Nullable columns can cause final results to be evaluated as NULL for the predicate.

I don't see the problem here. The error is related to performance; MSDN says I should use ISNULL() -- but an equals comparison against NULL is always false, right? Am I missing something, or is the warning just wrong?

Answer 1

I think it's referring to the WHERE clause. It's saying that both your parameter and your column can be NULL, in which case your WHERE clause no longer evaluates to true/false. By funneling your nullable column into one that always has a value defined (via ISNULL), you're in better shape, logic-wise.

Here's the Microsoft documentation on that error.

On the aside, NULLs supposedly make queries a skosh slower.

Answer 2

I think the analyzer might just not be taking into account your IF statement.

Your code seems correct to me.

Answer 3

I think it's a spurious warning - can you suppress it on a case-by-case basis, or that particular warning completely?

What happens when you do this?:

CREATE TABLE MyTable (
  Id   UNIQUEIDENTIFIER NOT NULL,
  Info VARCHAR(MAX)
)

Answer 4

Null comparison depends on setup.

When SET ANSI_NULLS is ON, all comparisons against a null value evaluate to UNKNOWN

When SET ANSI_NULLS is ON, a SELECT statement that uses WHERE column_name = NULL returns zero rows even if there are null values in column_name. A SELECT statement that uses WHERE column_name <> NULL returns zero rows even if there are no nnull values in column_name.

When SET ANSI_NULLS is OFF, the Equals (=) and Not Equal To (<>) comparison operators do not follow the ISO standard.

This is from here.

Answer 5

IF @id IS NOT NULL

should be replaced with

IF ISNull(@id, -1) <> -1

Answer 6

@ Raj: "IF ISNull(@id, -1) <> -1"

I would not do it as it actually replaces the table entry