自定义变量jsonString，substring_index(@jsonString, '"},"', 1),截取正数第1个"},"之前的全部字符；substring_index(substring_index(@jsonString, '"},"', 1) , ':"', -1)，截取倒数数第1个"},"之后的全部字符；【可以直接执行，@jsonString是要截取的字段】
set @jsonString= '[{"score":{"min":"0","max":"60"},"code":"差"},{"score":{"min":"60","max":"80"},"code":"中"},{"score":{"min":"80","max":"90"},"code":"良"},{"score":{"min":"90","max":"100"},"code":"优"}]';
SELECT
@jsonString,
substring_index(@jsonString, '"},"', 1),
substring_index(substring_index(@jsonString, '"},"', 1) , ':"', -1)