Introduction

To validate something as being possible, you can use the IS operator. For example, to acknowledge that something is NULL, you can use the IS NULL expression. Here is an example:

-- Square Calculation
DECLARE @Side As Decimal(10,3),
        @Perimeter As Decimal(10,3),
        @Area As Decimal(10,3);

SET     @Perimeter = @Side * 4;
SET     @Area = @Side * @Side;
IF @Side IS NULL
	PRINT 'A null value is not welcome'
ELSE IF @Side > 0
    BEGIN
	SELECT @Side AS Side;
	SELECT @Perimeter AS Perimeter ;
	SELECT @Area AS Area;
    END;
ELSE
	PRINT N'You must provide a positive value';
GO

This would produce:

To avoid having a NULL value, you can either initialize the variable or you can assign it a value. Here is an example:

-- Square Calculation
DECLARE @Side As Decimal(10,3),
        @Perimeter As Decimal(10,3),
        @Area As Decimal(10,3);
SET     @Side = 48.126;
SET     @Perimeter = @Side * 4;
SET     @Area = @Side * @Side;
IF @Side IS NULL
	PRINT N'A null value is not welcome'
ELSE IF @Side > 0
    BEGIN
		SELECT @Side AS Side;
		SELECT @Perimeter AS Perimeter ;
		SELECT @Area AS Area;
    END;
ELSE
	PRINT 'You must provide a positive value';
GO

This would produce: