In MSSQL What is ISNULL() function? how do we use it?

Answer:

ISNULL() function is used to check the value is null or not in SQL Server. This
function also provides a way to replace a value with the null if the result is true.

Here @Param is a nullable parameter and isnull checks the parameter is a null value
or not. If it is a null, it replaces it with '' string.

ISNULL(@Param, '')