MySQL in a Nutshell, 2nd Edition

Name

NULLIF()

Synopsis

NULLIF(condition1, condition2)

This function returns NULL if the two arguments given are equal. Otherwise, it returns the value or results of the first argument. Here is an example:

SELECT clients.client_id AS ID,
CONCAT(name_first, SPACE(1), name_last) AS Client,
telephone_home AS Telephone,
NULLIF(
  (SELECT SUM(qty * price)
   FROM investments, stock_prices
   WHERE stock_symbol = symbol
   AND client_id = ID ), 0)
AS Value
FROM clients, investments
WHERE clients.client_id = investments.client_id
GROUP BY clients.client_id;

In this example, NULL is returned for the Value column if the value of the client’s stocks is 0 (i.e., the client had stocks but sold them all). If there is a value to the stocks, however, the sum of their values is displayed.