How can I test an IF statement in MySQL?

How can I test an IF statement in MySQL ?

I need to know, what happens when a query is into a IF statement and it returns nothing (0 row selected) ?

In other word, what a SELECT query returns when the no row is selected? Will it return NULL? If yes, so will this throw an error? IF ( NULL ) THEN ...? Or in this case just the statement of that IF won't execute?

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In reality, I have a code like this:

IF ( SELECT banned FROM users WHERE id=new.user_id ) THEN 
// do stuff

Note: banned column contains either 0 or 1.

All I want to know, is my code safe when that query doesn't return any row? I mean will it throw an error or nothing happens (just that IF statement doesn't execute) ?

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Noted that I wanted to test it myselft, but I don't know how can I test a IF statement in MySQL. For example, I wanted to test it by this code:

BEGIN
    IF ( SELECT banned FROM users WHERE id=new.user_id ) THEN 
       SELECT 'test';
    ENDIF;
END

But code above doesn't even run. That's why I've asked this question.