Luhn Algorithm in Teradata SQL

Luhn algorithm is used, among others, to calculate the checksum digit of credit cards and mobile handset IMEIs. The following is my attempt to implement this algorithm in Teradata sql. It flags each IMEI as valid or not. Needless to say, IMEIs would typically be read from a table rather than hard-coded as in this example.

SELECT dt3.IMEI
       ,CASE 
          WHEN (dt3.dig1 + dt3.dig2 + dt3.dig3 + dt3.dig4 
           + dt3.dig5 + dt3.dig6 + dt3.dig7
           + dt3.dig8 + dt3.dig9 + dt3.dig10 + dt3.dig11 
           + dt3.dig12 + dt3.dig13 
           + dt3.dig14 + dt3.dig15) MOD 10 = 0 THEN 'Y' 
          ELSE 'N'
       END AS VALID_IMEI          
FROM
(
SELECT dt2.IMEI
    ,dt2.dig1
    ,CASE 
            WHEN dt2.dig2 = 0 THEN 0
            WHEN dt2.dig2 MOD 9 = 0 THEN 9
            ELSE dt2.dig2 MOD 9
     END AS dig2
    ,dt2.dig3
    ,CASE 
            WHEN dt2.dig4 = 0 THEN 0
            WHEN dt2.dig4 MOD 9 = 0 THEN 9
            ELSE dt2.dig4 MOD 9
     END AS dig4
    ,dt2.dig5
   ,CASE 
            WHEN dt2.dig6 = 0 THEN 0
            WHEN dt2.dig6 MOD 9 = 0 THEN 9
            ELSE dt2.dig6 MOD 9
   END AS dig6
   ,dt2.dig7
   ,CASE 
            WHEN dt2.dig8 = 0 THEN 0
            WHEN dt2.dig8 MOD 9 = 0 THEN 9
            ELSE dt2.dig8 MOD 9
   END AS dig8
   ,dt2.dig9
   ,CASE 
            WHEN dt2.dig10 = 0 THEN 0
            WHEN dt2.dig10 MOD 9 = 0 THEN 9
            ELSE dt2.dig10 MOD 9
   END AS dig10
   ,dt2.dig11
   ,CASE 
            WHEN dt2.dig12 = 0 THEN 0
            WHEN dt2.dig12 MOD 9 = 0 THEN 9
            ELSE dt2.dig12 MOD 9
   END AS dig12
   ,dt2.dig13
   ,CASE 
            WHEN dt2.dig14 = 0 THEN 0
            WHEN dt2.dig14 MOD 9 = 0 THEN 9
            ELSE dt2.dig14 MOD 9
   END AS dig14
   ,dt2.dig15
FROM
(
SELECT dt1.IMEI
        ,SUBSTR(dt1.IMEI, 1, 1) AS dig1
        ,SUBSTR(dt1.IMEI, 2, 1) * 2 AS dig2
        ,SUBSTR(dt1.IMEI, 3, 1) AS dig3
        ,SUBSTR(dt1.IMEI, 4, 1)  * 2 AS dig4
        ,SUBSTR(dt1.IMEI, 5, 1) AS dig5
        ,SUBSTR(dt1.IMEI, 6, 1)  * 2 AS dig6
        ,SUBSTR(dt1.IMEI, 7, 1) AS dig7
        ,SUBSTR(dt1.IMEI, 8, 1)  * 2 AS dig8
        ,SUBSTR(dt1.IMEI, 9, 1) AS dig9
        ,SUBSTR(dt1.IMEI, 10, 1) * 2 AS dig10
        ,SUBSTR(dt1.IMEI, 11, 1) AS dig11
        ,SUBSTR(dt1.IMEI, 12, 1) * 2 AS dig12
        ,SUBSTR(dt1.IMEI, 13, 1) AS dig13
        ,SUBSTR(dt1.IMEI, 14, 1) * 2 AS dig14
        ,SUBSTR(dt1.IMEI, 15, 1) AS dig15
FROM
(
SELECT '999999999999999' AS IMEI
FROM SYS_CALENDAR.CALENDAR
WHERE calendar_date = CURRENT_DATE

UNION 

SELECT '352651010278244' AS IMEI
FROM SYS_CALENDAR.CALENDAR
WHERE calendar_date = CURRENT_DATE

) AS dt1
) AS dt2
) AS dt3
;

It returns the following answerset: