Joe Celko's SQL for Smarties, 3rd Edition

6.7. Functions and NULLs

Most vendors propagate NULLs in the functions they offer as extensions of the standard ones required in SQL. For example, the cosine of a NULL will be NULL. There are two functions that convert NULLs into values:

1.	NULLIF (V1, V2) returns a NULL when the first parameter equals the second parameter. The function is equivalent to the following case specification: CASE WHEN (V1 = V2) THEN NULL ELSE V1 END
2.	COALESCE (V1, V2, V3, ..., Vn) processes the list from left to right and returns the first parameter that is not NULL. If all the values are NULL, it returns a NULL.