IF Statement In SELECT WHERE Clause

Hi Everyone,

I have the following stored procedure, I would like to use
IF statement or something of the sort in the where clause i.e.
The last line in the SP is: AND (category.categoryID = @categoryID),
I only want to check this, if @categoryID is not = 12.
So can I do something like this:

IF @categoryID <> 12
AND (category.categoryID = @categoryID)

STORED PROCEDURE:

CREATE PROCEDURE sp_get_total_risk_patients
@categoryID int
AS

SELECT COUNT(DISTINCT patient.patientID) AS total_patients
FROM patient
INNER JOIN patient_record ON patient.patientID = patient_record.patientID
INNER JOIN sub_category ON sub_category.sub_categoryID = patient.sub_categoryID
INNER JOIN category ON category.categoryID = sub_category.categoryID
WHERE risk = 6
AND (completed_date = '' OR completed_date IS NULL)
AND (category.categoryID = @categoryID)