SQL Server stored procedure with case - sql-server

Question

I am trying to create a stored procedure in SQL Server 2019 where you should insert an employee number and an amount of salary increase by the keyboard.
You should add the amount to the salary if the employee number does not have any commission.
If he has some commission then you should show an error message. You should also show an error message if the employee number does not exist in the table
I am trying to solve this exercise with a case structure but I am not able to do it
Can someone help me?
Thanks in advance
CREATE PROCEDURE subir_sueldo
#emp int,
#com int
AS
BEGIN
SELECT EMP_NO
FROM EMPLE
CASE
WHEN COMISION IS NULL
THEN
UPDATE EMPLE
SET SALARIO = SALARIO + #com
WHERE EMP_NO = #emp
WHEN COMISION IS NOT NULL
THEN
PRINT 'the comission is not null '
WHEN #emp NOT IN (SELECT #emp FROM EMPLE WHERE DEPT_NO = #emp)
THEN
PRINT 'user does not exist'
ELSE
BREAK
END
END

Answer 1

CASE in T-SQL/SQL Server is an expression that returns one of several possible, atomic, single values - it is NOT a flow control statement ..... for that, you need to use IF ... ELSE ... statements - something like this:
CREATE PROCEDURE subir_sueldo
#emp INT,
#com INT
AS
BEGIN
-- you didn't show or tell what datatype "Commission"
-- (should be with TWO "m" and "s" in English) is - just guessing here!
DECLARE #Commission DECIMAL(20,4);
SELECT #Commission = COMMISSION
FROM EMPLE
WHERE EMP_NO = #emp;
IF (#Commission IS NULL)
UPDATE EMPLE
SET SALARIO = SALARIO + #com
WHERE EMP_NO = #emp
ELSE
PRINT 'The commission is not null '
-- you would probably want to check this *FIRST* and stop the
-- procedure if your #emp doesn't match an existing user.....
IF #emp NOT IN (SELECT #emp FROM EMPLE WHERE DEPT_NO = #emp)
PRINT 'user does not exist'
ELSE
BREAK
END

Answer 2

As an alternative idea, you could do something like this, that tries to UPDATE the table, and then if it reports no rows were updated, let's the user know; though this will not give different errors for if the user does not exist or if they have a commission (though sometimes ambiguity is good).
CREATE PROCEDURE dbo.subir_sueldo #emp int, #com int AS
BEGIN
SET NOCOUNT ON;
UPDATE dbo.EMPLE
SET SALARIO = SALARIO +#Com
WHERE EMP_NO = #emp
AND COMISION IS NULL;
IF ##ROWCOUNT = 0
--I use THROW as it seems like you actually want an error
--Use an error number appropriate for your environment.
THROW 65489, N'User does not exist, or Commission is not NULL.',10;
END;