来自MySQL文档：
“CONV（）使用64位精度”
这意味着超过64位的值将被截断：

select (conv("19d971e4fe8401e74000000",16,10))\G
*************************** 1. row ***************************
(conv("19d971e4fe8401e74000000",16,10)): 18446744073709551615
1 row in set, 1 warning (0,001 sec)

show warnings\G
*************************** 1. row ***************************
  Level: Warning
   Code: 1292
Message: Truncated incorrect DECIMAL value: '19d971e4fe8401e74000000'
1 row in set (0,001 sec)

select hex(18446744073709551615)\G
*************************** 1. row ***************************
hex(18446744073709551615): FFFFFFFFFFFFFFFF
1 row in set (0,001 sec)

SET @val = '19d971e4fe8401e74000000';

WITH RECURSIVE
cte AS (
  SELECT CONV(RIGHT(@val, 8), 16, 10) part,
         LEFT(@val, GREATEST(LENGTH(@val) - 8, 0)) rest,
         CAST(1 AS DECIMAL(40,0)) multiplier
  UNION ALL
  SELECT CONV(RIGHT(rest, 8), 16, 10),
         LEFT(rest, GREATEST(LENGTH(rest) - 8, 0)),
         CAST(multiplier AS DECIMAL(40,0)) * CAST(POW(2, 32) AS DECIMAL(40,0))
  FROM cte
  WHERE rest <> ''
)
SELECT SUM(CAST(CAST(part AS DECIMAL(40,0)) * CAST(multiplier AS DECIMAL(40,0)) AS DECIMAL(40,0))) result
FROM cte

| 结果|
| - ------|
| 500亿美元|
fiddle
当然，有些CAST可能是多余的。DECIMAL(40,0)可能会稍微扩展。