CREATE TABLE Wages
(
emp_id tinyint identity,
hourly_wage decimal NULL,
salary decimal NULL,
commission decimal NULL,
num_sales tinyint NULL
)

insert into wages(hourly_wage) values ( 8 )
insert into wages(hourly_wage) values ( 9 )
insert into wages(hourly_wage) values ( 10 )

insert into wages(salary) values ( 1000 )
insert into wages(salary) values ( 1100 )
insert into wages(salary) values ( 1200 )

insert into wages(commission,num_sales) values ( 10 , 10 )
insert into wages(commission,num_sales) values ( 15 , 20 )
insert into wages(commission,num_sales) values ( 20 , 30 )

select * from wages

 1 . Даёт NULL везде

SELECT CAST (hourly_wage *  40  *  52  +
salary +
commission * num_sales AS MONEY) as YearlyWages
FROM Wages

 2 . Даёт результат
SELECT CAST (COALESCE (hourly_wage *  40  *  52 ,
Salary,
commission * num_sales) AS MONEY) as YearlyWages
FROM Wages

 3 . Line  2 : Incorrect syntax near ','. 
SELECT CAST (CASE
WHEN((hourly_wage,) IS NOTNULL) THEN hourly_wage *  40  *  52 
WHEN(NULLIF(salary,NULL)IS NULL)THEN salary
ELSE commission * num_sales
END
AS MONEY)
As_yearlyWages
FROM Wages

 4 . Не считает для  1 - 3  и  7 - 9 
SELECT CAST(CASE
WHEN (hourly_wage IS NULL)THEN salary
WHEN (salary IS NULL)THEN commission*num_sales
ELSE commission * num_sales
END
AS MONEY)
As YearlyWages
FROM Wages