Question

I’d like to select data from a table with the following rule, but I’m having trouble writing the query. I’m using PostgreSQL and I can’t create a UDF. The table looks like this:

id  | user_id  | account_number
-------------------------------
 1  |    1     | 12345671
 2  |    4     | 12356673
 3  |    7     | 12325678

The id and user_id are integers whereas the account number is a string. I’d like to select the account numbers that match the following conditions:

• Account number string contains exactly 8 digits
• Validation scheme
  1. Take the first 7 digits
  2. Multiply the first digit by 1, the second by 2, the third by 3, the fourth by 1, the fifth by 2, the sixth by 3 and the seventh by 1
  3. Sum the result of multiplying each digit by the relevant number
  4. If the 8th digit is the same as mod(sum, 10) then select this number

In this table above, I should only return the first two rows with the query.

Just to repeat, I can’t create a UDF, so am looking to find out whether this is possible using ordinary SQL in a query.

Thanks!

Answer 1

Yes, you can do it. Basically, use SIMILAR TO to check for exactly 8 digits, then substring and cast to do the math. Something like this:

SELECT * FROM table_name WHERE
  account_number SIMILAR TO '[0-9]{8}'
  AND (
    1 * CAST(SUBSTR(account_number, 1, 1) AS INTEGER) +
    2 * CAST(SUBSTR(account_number, 2, 1) AS INTEGER) +
    3 * CAST(SUBSTR(account_number, 3, 1) AS INTEGER) +
    1 * CAST(SUBSTR(account_number, 4, 1) AS INTEGER) +
    2 * CAST(SUBSTR(account_number, 5, 1) AS INTEGER) +
    3 * CAST(SUBSTR(account_number, 6, 1) AS INTEGER) +
    1 * CAST(SUBSTR(account_number, 7, 1) AS INTEGER)
  )%10 = CAST(SUBSTR(account_number, 8, 1) AS INTEGER)

Of course, this returns no rows in your example, because:

1×1 + 2×2 + 3x3 + 1×4 + 2×5 + 3×6 + 1×7 = 53
53 MOD 10 = 3
3 ≠ 1

PS: You do realize the UDFs can be written in languages other than C. E.g., you can write one in PL/pgSQL.