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 This is because NULL has been replaced by 100 via the ISNULL function.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

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