Question

I’ve been looking to find a good way to do a SQL query where I can have a where statement which, if left blank, will act like it wasn’t there at all. I’ve found this, which seems to work quite well:

WHERE (Column = @value OR @value is null)

If I specify a value for @value, the search is filtered like I want. But if I pass in null, it’s like saying @value can be anything. I like this. This is good. What I don’t understand though is, why does this work like this?

Answer 1

If @value is null, your WHERE clause:

WHERE (Column = @value OR @value is null)

reduces to

WHERE (Column = @value OR 1=1)

(This is similar to if (Column == value || true) in other common languages)

An OR conjunction is true if either of its operands (sides) are true: SQL uses three valued logic

+---------+------+---------+---------+
| A OR B  | TRUE | Unknown |  FALSE  |
+---------+------+---------+---------+
| TRUE    | TRUE | TRUE    | TRUE    |
| Unknown | TRUE | Unknown | Unknown |
| FALSE   | TRUE | Unknown | FALSE   |
+---------+------+---------+---------+

And so:

• If @value is null, the right side of your WHERE clause is true, so the entire conditional is true, and your WHERE clause will always be satisfied.
• If @value isn’t null, then the right side of your WHERE clause is false, then whether the WHERE clause is satisfied depends on whether Column = @value.