Returns the first argument that is not null. All arguments must be compatible. IFNULL requires two arguments. COALESCE and VALUE can have any number, two or greater.
Example:
When selecting all the values from all the rows in the DEPARTMENT table, if the department manager (MGRNO) is missing (that is, null), then return a value of 'ABSENT'.
SELECT DEPTNO, DEPTNAME, COALESCE(MGRNO, 'ABSENT'), ADMRDEPT FROM DEPARTMENT