The ISNULL function and the COALESCE expression have a similar purpose but ISNULL can have two parameters only, COALESCE can have multiple input.
Both are checking the parameters in order and returns the current value of the first expression that initially doesn’t evaluate to NULL.
SELECT ISNULL(hourly_wage * 40 * 52, salary) AS income
SELECT COALESCE(hourly_wage * 40 * 52, salary, commission) AS income
The NULLIF function returns a NULL value if the two parameters are equal. for instance, we could use NULLIF if we wanted to return NULLs whenever the field equal giving value:
SELECT NULLIF(4,4) AS Same, NULLIF(5,7) AS Different;