How to compare values which may both be null in T-SQL

Use INTERSECT operator.

It's NULL-sensitive and efficient if you have a composite index on all your fields:

IF      EXISTS
        (
        SELECT  MY_FIELD1, MY_FIELD2, MY_FIELD3, MY_FIELD4, MY_FIELD5, MY_FIELD6
        FROM    MY_TABLE
        INTERSECT
        SELECT  @IN_MY_FIELD1, @IN_MY_FIELD2, @IN_MY_FIELD3, @IN_MY_FIELD4, @IN_MY_FIELD5, @IN_MY_FIELD6
        )
BEGIN
        goto on_duplicate
END

Note that if you create a UNIQUE index on your fields, your life will be much simpler.

Use ISNULL:

ISNULL(MY_FIELD1, 'NULL') = ISNULL(@IN_MY_FIELD1, 'NULL')

You can change 'NULL' to something like 'All Values' if it makes more sense to do so.

It should be noted that with two arguments, ISNULL works the same as COALESCE, which you could use if you have a few values to test (i.e.-COALESCE(@IN_MY_FIELD1, @OtherVal, 'NULL')). COALESCE also returns after the first non-null, which means it's (marginally) faster if you expect MY_FIELD1 to be blank. However, I find ISNULL much more readable, so that's why I used it, here.

IF EXISTS(SELECT * FROM MY_TABLE WHERE 
            (MY_FIELD1 = @IN_MY_FIELD1 
                     or (MY_FIELD1 IS NULL and @IN_MY_FIELD1 is NULL))  AND
            (MY_FIELD2 = @IN_MY_FIELD2 
                     or (MY_FIELD2 IS NULL and @IN_MY_FIELD2 is NULL))  AND
            (MY_FIELD3 = @IN_MY_FIELD3 
                     or (MY_FIELD3 IS NULL and @IN_MY_FIELD3 is NULL))  AND
            (MY_FIELD4 = @IN_MY_FIELD4 
                     or (MY_FIELD4 IS NULL and @IN_MY_FIELD4 is NULL))  AND
            (MY_FIELD5 = @IN_MY_FIELD5 
                     or (MY_FIELD5 IS NULL and @IN_MY_FIELD5 is NULL))  AND
            (MY_FIELD6 = @IN_MY_FIELD6
                     or (MY_FIELD6 IS NULL and @IN_MY_FIELD6 is NULL)))
            BEGIN
                    goto on_duplicate
            END

Wordy As compared to the IFNULL/COALESCE solution. But will work without having to think about what value will not appear in the data that can be used as the stand in for NULL.

You could coalesce each value, but it's a bit wince-inducing:

    IF EXISTS(SELECT * FROM MY_TABLE WHERE 
    coalesce(MY_FIELD1,'MF1') = coalesce(@IN_MY_FIELD1,'MF1')  AND
    ...
    BEGIN
            goto on_duplicate
    END

You'd also need to ensure that the coalesced value is not an otherwise valid value on the column in question. For example, if it was possible that the value of MY_FIELD1 could be 'MF1' then this would cause a lot of spurious hits.

Equals comparison:

((f1 IS NULL AND f2 IS NULL) OR (f1 IS NOT NULL AND f2 IS NOT NULL AND f1 = f2))

Not Equal To comparison: Just negate the Equals comparison above.

NOT ((f1 IS NULL AND f2 IS NULL) OR (f1 IS NOT NULL AND f2 IS NOT NULL AND f1 = f2))

Is it verbose? Yes, it is. However it's efficient since it doesn't call any function. The idea is to use short circuit in predicates to make sure the equal operator (=) is used only with non-null values, otherwise null would propagate up in the expression tree.

You create a primary key on your fields and let the engine enforce the uniqueness. Doing IF EXISTS logic is incorrect anyway as is flawed with race conditions.

You could use SET ANSI_NULLS in order to specify the behavior of the Equals (=) and Not Equal To (<>) comparison operators when they are used with null values.

Above, @drowa shows a verbose approach that I agree with. It's good because it avoids the 3-value logic problem. Many of the other approaches provided here will fail in subtle and unexpected ways when negated because they're treating null as equivalent to false which it is not.

However, I have a workflow that I find makes it convenient-ish, here's a regex. Given code of the form

(leftSide <=> rightSide)

regex find this:

\(([a-zA-Z0-9_.@]+)\s*<=>\s*([a-zA-Z0-9_.@]+)\)

and replace with this:

(/*$1 <=> $2*/ ($1 IS NULL AND $2 IS NULL) OR ($1 IS NOT NULL AND $2 IS NOT NULL AND $1 = $2))

So I write the (leftSide <=> rightSide) code and apply the above regex transformation to get the expanded form. It'd be nicer if MSSQL offered some kind of macro expansion so I wouldn't have to do it manually, but it doesn't.

@Drowa's answer quoted for reference:

Equals comparison:

((f1 IS NULL AND f2 IS NULL) OR (f1 IS NOT NULL AND f2 IS NOT NULL AND f1 = f2))

Not Equal To comparison: Just negate the Equals comparison above.

NOT ((f1 IS NULL AND f2 IS NULL) OR (f1 IS NOT NULL AND f2 IS NOT NULL AND f1 = f2))

Is it verbose? Yes, it is. However it's efficient since it doesn't call any function. The idea is to use short circuit in predicates to make sure the equal operator (=) is used only with non-null values, otherwise null would propagate up in the expression tree.

You will have to use IS NULL or ISNULL. There really isn't a away around it.

From SQL Server 2022 the "null safe" version of

WHERE 
    MY_FIELD1 = @IN_MY_FIELD1  AND
    MY_FIELD2 = @IN_MY_FIELD2  AND
    MY_FIELD3 = @IN_MY_FIELD3  AND 
    MY_FIELD4 = @IN_MY_FIELD4  AND
    MY_FIELD5 = @IN_MY_FIELD5  AND
    MY_FIELD6 = @IN_MY_FIELD6

would be

WHERE 
    MY_FIELD1 IS NOT DISTINCT FROM @IN_MY_FIELD1  AND
    MY_FIELD2 IS NOT DISTINCT FROM @IN_MY_FIELD2  AND
    MY_FIELD3 IS NOT DISTINCT FROM @IN_MY_FIELD3  AND 
    MY_FIELD4 IS NOT DISTINCT FROM @IN_MY_FIELD4  AND
    MY_FIELD5 IS NOT DISTINCT FROM @IN_MY_FIELD5  AND
    MY_FIELD6 IS NOT DISTINCT FROM @IN_MY_FIELD6

IS [NOT] DISTINCT FROM (Transact-SQL)

Did you check NULLIF? http://msdn.microsoft.com/en-us/library/ms177562.aspx