Как разбить каждую цифру числа на разные строки

SELECT
    SUBSTR(3987654321, idx, 1) AS SplitDigits,
    3 * (SUBSTR(3987654321, idx, 1)) AS ConstTimesSplitDigits
   -- SUM(SUBSTR(2050364496, idx, 1) * (SUBSTR(5432765432, idx, 1))) AS Result
FROM (SELECT @cnt := 0) A
-- cross join any table has rows greater than or equal to your liking
CROSS JOIN (SELECT (@cnt := @cnt + 1) idx FROM INFORMATION_SCHEMA.TABLES LIMIT 10) B
;

Then I have to sum all the multiplications. After that there's some trivial calculations with the result of the sum, applying some modulus and complements, but it's out of the scope of this question.

Почему за рамками этого вопроса, ведь мы любим решать эти вопросы.

Запрос

SELECT 
 11 - (SUM(vector_input.vector_input__summed) % 11)
FROM (

SELECT 
 vector.item * input.item AS vector_input__summed

FROM (
 SELECT 
  DISTINCT
    SUBSTRING(5432765432, vector_number_generator.number, 1) AS item
    ,  vector_number_generator.number  AS position
FROM (
  SELECT 
   @vector_row := @vector_row + 1 AS number
  FROM (
    SELECT 0 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION   SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9
  ) AS row_1
  CROSS JOIN (
    SELECT @vector_row := 0 
  ) AS init_user_params 
) AS vector_number_generator

) AS vector
INNER JOIN (
  SELECT 
  DISTINCT
       SUBSTRING(2050364496, number_generator.number, 1) AS item
    ,  number_generator.number  AS position
FROM (
  SELECT 
   @row := @row + 1 AS number
  FROM (
    SELECT 0 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION   SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9
  ) row1
  CROSS JOIN (
    SELECT 0 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION  SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9
  ) row2
  CROSS JOIN (
    SELECT @row := 0 
  ) AS init_user_params 
) AS number_generator
) AS input
ON
 vector.position = input.position
) AS vector_input

Результат

| 11 - (SUM(vector_input.vector_input__summed) % 11) |
| -------------------------------------------------- |
| 8                                                  |

См. демо

Если также следует использовать модуль 10, что, как я предполагаю, должно быть
((11 - (SUM(vector_input.vector_input__summed) % 11)) % 10) см. демо

Поскольку формат числа фиксирован, вам просто нужна базовая математика:

SELECT num div          10 % 10 * 2
     + num div         100 % 10 * 3
     + num div        1000 % 10 * 4
     + num div       10000 % 10 * 5
     + num div      100000 % 10 * 6
     + num div     1000000 % 10 * 7
     + num div    10000000 % 10 * 2
     + num div   100000000 % 10 * 3
     + num div  1000000000 % 10 * 4
     + num div 10000000000 % 10 * 5 AS s
FROM (
    SELECT 20503644968 AS num
) AS t
-- 157

Использование ответов Наэ и Рэймонда

Чтобы разбить число фиксированной длины на отдельные строки

SELECT
   SUBSTR(1234567890123, range10, 1)
FROM (
  SELECT 1 range10
  UNION SELECT 2
  UNION SELECT 3
  UNION SELECT 4
  UNION SELECT 5
  UNION SELECT 6
  UNION SELECT 7
  UNION SELECT 8
  UNION SELECT 9
  UNION SELECT 10
  UNION SELECT 11
  UNION SELECT 12
  UNION SELECT 13
) A

Количество символов переменной длины см. в ответе Наэ.

Вся проверка RUC превращена в функцию

CREATE FUNCTION `validate_ruc`(`ruc` BIGINT) RETURNS binary(1)
BEGIN
DECLARE isValid BINARY;

IF LENGTH(ruc) != 11 THEN RETURN FALSE; END IF;

SELECT
  ((11 - (sum % 11)) % 10) = (ruc % 10)
INTO isValid
FROM (
  SELECT
     SUM( SUBSTR(ruc, range10, 1) * (SUBSTR(5432765432, range10, 1)) ) AS sum
  FROM (
    SELECT 1 range10
    UNION SELECT 2
    UNION SELECT 3
    UNION SELECT 4
    UNION SELECT 5
    UNION SELECT 6
    UNION SELECT 7
    UNION SELECT 8
    UNION SELECT 9
    UNION SELECT 10
  ) A
) B;

RETURN isValid;

END