写个自定义函数。 假设，a b 严格用逗号(,)隔开

delimiter $$
DROP FUNCTION IF EXISTS `my_add` $$
create function my_add(_str varchar(250))returns DECIMAL
begin
       declare pos DECIMAL;
       declare num DECIMAL;
       declare re DECIMAL;
       set num = 0;
       set re = 0;
       set pos = locate(',',_str);

       loop1: while pos > 0 do
           set num = cast(left(_str,pos-1) as DECIMAL);

           set re = re + num;
           set _str = SUBSTRING(_str,pos+1);
           set pos = locate(',',_str);

       end while loop1;
       set num = cast(_str as DECIMAL);
       set re = re + num;
       return(re);
end$$

select my_add('1,2,3,4,5')$$
# output 15