Luhn Algorithm

Use this to check if Luhn has valid check digitCREATE FUNCTIONdbo.fnIsLuhnValid
(
@Luhn VARCHAR(8000)
)
RETURNS BIT
AS

BEGIN
IF @Luhn LIKE '%[^0-9]%'
RETURN 0

DECLARE@Index SMALLINT,
@Multiplier TINYINT,
@Sum INT,
@Plus TINYINT

SELECT@Index = LEN(@Luhn),
@Multiplier = 1,
@Sum = 0

WHILE @Index >= 1
SELECT@Plus = @Multiplier * CAST(SUBSTRING(@Luhn, @Index, 1) AS TINYINT),
@Multiplier = 3 - @Multiplier,
@Sum = @Sum + @Plus / 10 + @Plus % 10,
@Index = @Index - 1

RETURN CASE WHEN @Sum % 10 = 0 THEN 1 ELSE 0 END
END
Peter Larsson
Helsingborg, Sweden