If else on WHERE clause

Question

I\'ve this query:

SELECT  `id` ,  `naam` 
FROM  `klanten` 
WHERE (
`email` LIKE  \'%@domain.nl%\'
OR  `email2` LIKE  \'%@domain.nl%\'
)

But

Answer 1

Here is a sample query for a table having a foreign key relationship to the same table with a query parameter.

SET @x = -1;
SELECT id, categoryName 
FROM Catergory WHERE IF(@x > 0,category_ParentId = @x,category_ParentId IS NOT NULL);

@x can be changed.