这个意思？

select if('10<20','ture','false');
+----------------------------+
| if('10<20','ture','false') |
+----------------------------+
| ture                       |
+----------------------------+
1 row in set, 1 warning (0.00 sec)

题主应该写错了。 是select ‘10’<‘20’ 吧。

写个存储过程，传入字符串，然后拼接动态sql，返回输出值

delimiter //
CREATE   PROCEDURE eval( str VARCHAR(50)) 
BEGIN
     set @sqltext := concat('select ',str);
     PREPARE stmt from @sqltext;
      EXECUTE stmt  ;
      DEALLOCATE PREPARE stmt;
end//
delimiter;

call  eval('1>2')