IFNULL() is a function used in MySQL servers which takes in two parameters, one being the field name and the other parameter is the value to return in case of null entries. Not null values are returned as it is, while the null values are represented by the value provided in the second parameter. For example, null values from the gender column can be replaced with the text ‘Prefer not to say’ in the result using the following query –  SELECT IFNULL(gender, “Prefer not to say”)
FROM employees; Similar functionality is present in other database servers but with different function names. Oracle servers have NVL(), SQL servers have ISNULL() and COALESCE() in PostgreSQL.