MySQL Cookbook

Mapping NULL Values to Other Values for Display

Problem

A query’s output includes NULL values, but you’d rather see something more meaningful, like “Unknown.”

Solution

Convert NULL values selectively to another value when displaying them. You can also use this technique to catch divide-by-zero errors.

Discussion

Sometimes it’s useful to display NULL values using some other distinctive value that has more meaning in the context of your application. If NULL id values in the taxpayer table mean “unknown,” you can display that label by using IF( ) to map them onto the string Unknown:

mysql> SELECT name, IF(id IS NULL,'Unknown', id) AS 'id' FROM taxpayer;
+---------+---------+
| name    | id      |
+---------+---------+
| bernina | 198-48  |
| bertha  | Unknown |
| ben     | Unknown |
| bill    | 475-83  |
+---------+---------+

Actually, this technique works for any kind of value, but it’s especially useful with NULL values because they tend to be given a variety of meanings: unknown, missing, not yet determined, out of range, and so forth.

The query can be written more concisely using IFNULL( ), which tests its first argument and returns it if it’s not NULL, or returns its second argument otherwise:

mysql> SELECT name, IFNULL(id,'Unknown') AS 'id' FROM taxpayer;
+---------+---------+
| name    | id      |
+---------+---------+
| bernina | 198-48  |
| bertha  | Unknown |
| ben     | Unknown |
| bill    | 475-83  |
+---------+---------+

In other words, these two tests are equivalent:

IF(expr1 IS NOT NULL,expr1,expr2) IFNULL( ...