Question

I have a question about case statements and nulls in a where clause.

I want to do the following:

Declare @someVar int = null

select column1 as a from
TestTable t
where column1 = case when @someVar is not null then @someVar else column1 end

Here is the problem:

Let’s say @someVar is null. Let’s also say that column1 from TestTable t has NULL column values. Then, my condition t = t in the case statement will always evaluate to false.

I basically just want to be able to conditionally filter the column based on the value of @someVar if it’s provided. Any help?

Answer 1

• If @someVar IS NULL always return the row.
• If @someVar IS NOT NULL and column1 = @someVar return the row.
• Otherwise don’t return the row.

So try this:

SELECT column1
FROM TestTable t
WHERE @someVar IS NULL OR column1 = @someVar

To test that this expression works try inserting some test values into a table and then fetch all the rows where b is NULL or a is equal to b:

CREATE TABLE Table1 (a VARCHAR(20) NULL, b VARCHAR(20) NULL);
INSERT Table1 (a, b) VALUES
(NULL, NULL),
('Foo', NULL),
(NULL, 'Foo'),
('Foo', 'Foo'),
('Bar', 'Foo');

SELECT *
FROM Table1
WHERE b IS NULL OR a = b

Result:

a     b
NULL  NULL
Foo   NULL
Foo   Foo