SELECT t.hex in_hex, t.result in_result, @result out_result
FROM (
    SELECT BENCHMARK(
        CHAR_LENGTH(i.hex), 
        (
          SELECT LEAST(@result := @result + 
              CASE MID(i.hex, CHAR_LENGTH(i.hex) - @rownum,  1 )
                WHEN 'f' THEN  15 
                WHEN 'e' THEN  14 
                WHEN 'd' THEN  13 
                WHEN 'c' THEN  12 
                WHEN 'b' THEN  11 
                WHEN 'a' THEN  10 
                         ELSE MID(i.hex, CHAR_LENGTH(i.hex) - @rownum,  1 ) +  0 
              END * POW( 16 , @rownum),
              @rownum := @rownum +  1 
            )
        )
      ), i.result, i.hex
    FROM (SELECT @rownum :=  0  rownum, @result :=  0  result) v,
      (SELECT HEX( 2174861921 ) hex, CONV(HEX( 2174861921 ),  16 ,  10 ) result) i
  ) t;

SELECT SUM(CASE SUBSTR(i.hex, LENGTH(i.hex) - LEVEL +  1 ,  1 ) 
    WHEN 'F' THEN  15 
    WHEN 'E' THEN  14 
    WHEN 'D' THEN  13 
    WHEN 'C' THEN  12 
    WHEN 'B' THEN  11 
    WHEN 'A' THEN  10 
             ELSE SUBSTR(i.hex, LENGTH(i.hex) - LEVEL +  1 ,  1 ) +  0 
  END * POWER( 16 , LEVEL -  1 )) result
FROM (SELECT '81A1C261' hex,  2174861921  result FROM dual) i
CONNECT BY LEVEL <= LENGTH(i.hex);

SELECT t.input i_input, t.output i_output, @result output
FROM (
    SELECT BENCHMARK(
        CHAR_LENGTH(i.input), 
        (
          SELECT GREATEST(@result := @result + 
              (INSTR(i.abc, MID(i.input, CHAR_LENGTH(i.input) - @rownum,  1 )) -  1 ) * POW(CHAR_LENGTH(i.abc), @rownum),
              LEAST( 0 , @rownum := @rownum +  1 )
            )
        )
      ), i.output, i.input
    FROM (SELECT @rownum :=  0  rownum, @result :=  0  result) v,
      (SELECT HEX( 2174861921 ) input, '0123456789abcdef' abc, CONV(HEX( 2174861921 ),  16 ,  10 ) output) i
  ) t;

//нужно считывать по строкам и парсить по '=' - т.к. было не интересно - код не привожу!

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Ference@habr's implementation returns right result: 0 0 0 0 5.98
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iSage@habr's implementation does not return right result
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Azilot@habr's implementation does not return right result
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