ISBN13 -> ISBN10

Here’s a handy function for converting a 13-digit ISBN to its 10-digit equivalent in SQL Server T-SQL.

CREATE FUNCTION TO_ISBN10 ( @isbn varchar(13) )
RETURNS VARCHAR(10)
AS
BEGIN
    DECLARE @isbn10 varchar(10);
    DECLARE @forwards int;
    DECLARE @backwards int;
    DECLARE @check int;
    DECLARE @thisval int;

    SET @isbn10 =substring(@isbn,4,9);
    SET @forwards = 1;
    SET @backwards = 10;
    SET @check = 0;

    WHILE @forwards <= 9
    BEGIN
        SET @thisval =cast(substring(@isbn10, @forwards, 1) AS INT)* @backwards;
        SET @check = @check + @thisval;
        SET @forwards = @forwards + 1;
        SET @backwards = @backwards - 1;
    END

    SET @check = 11 -(@check % 11);

    IF @check = 10 begin
        SET @isbn10 = @isbn10 +'X';
    END
    ELSE BEGIN
        SET @isbn10 = @isbn10 +cast(@check AS VARCHAR(1));
    END;

    RETURN @isbn10;
END