SQL ISNULL Function

The ISNULL function is available in both SQL Server and MySQL. However, their uses are different:

SQL Server

In SQL Server, the ISNULL( ) function is used to replace NULL value with another value.

For example, if we have the following table,

Table Sales_Data

store_name	Sales
Store A	300
Store B	NULL

The following SQL,

SELECT SUM(ISNULL(Sales,100)) FROM Sales_Data;

returns 400. This is because NULL has been replaced by 100 via the ISNULL function.

MySQL

In MySQL, the ISNULL( ) function is used to test whether an expression is NULL. If the expression is NULL, this function returns 1. Otherwise, this function returns 0.

For example,

ISNULL(3*3) returns 0

ISNULL(3/0) returns 1