Both IFNULL and ISNULL are used to test whether an expression is null or not.

IFNULL: It follows the syntax: IFNULL(expr1,expr2), and it returns expression 1 if the value is null and expression 2 if it is not. For example, IFNULL([Sales],0) will return the value of Sales where it is not null, else return 0.

ISNULL: ISNULL follows the syntax: ISNULL(expr). It returns TRUE if the expression is NULL. For example, ISNULL([Sales]) will return true if Sales is null.