在MySQL Bugs上提交了 https://bugs.mysql.com/bug.php?id=84935 . 已经在MySQL8.0.1中修复
重现步骤
-- Prepare the table and populate it with records CREATE TABLE `voucher` ( `id` varchar(32) NOT NULL COMMENT 'ID', `vals` mediumtext NOT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; INSERT INTO `voucher` VALUES ('1', '{"period":90,"amount":5.45}'); INSERT INTO `voucher` VALUES ('2', '{"period":90,"amount":3.99}'); -- Execute queries mysql> SELECT vals FROM voucher WHERE id='1'; +-----------------------------+ | vals | +-----------------------------+ | {"period":90,"amount":5.45} | +-----------------------------+ 1 row in set (0.00 sec) mysql> SELECT json_extract(vals, '$.amount') FROM voucher WHERE id='1'; +--------------------------------+ | json_extract(vals, '$.amount') | +--------------------------------+ | 5.45 | +--------------------------------+ 1 row in set (0.00 sec) -- The result of this one is not correct mysql> SELECT SUM(json_extract(vals, '$.amount')) FROM voucher WHERE id='1'; +-------------------------------------+ | SUM(json_extract(vals, '$.amount')) | +-------------------------------------+ | 5 | +-------------------------------------+ 1 row in set (0.00 sec) mysql> SELECT ROUND(SUM(json_extract(vals, '$.amount')), 2) FROM voucher WHERE id='1'; +-----------------------------------------------+ | ROUND(SUM(json_extract(vals, '$.amount')), 2) | +-----------------------------------------------+ | 5.45 | +-----------------------------------------------+ 1 row in set (0.00 sec)