DROP TABLE IF EXISTS `sc`;
CREATE TABLE `sc` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(200) CHARACTER SET utf8 DEFAULT NULL,`class` varchar(200) CHARACTER SET utf8 DEFAULT NULL,`score` int(11) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=latin1;insert into sc values (1,'badkano','一年一班',100);
insert into sc values (2,'百度知道团长','一年一班',99);
insert into sc values (3,'du小短','一年一班',95);
insert into sc values (4,'du小小动','一年一班',97);
insert into sc values (5,'du小智','一年一班',80);
insert into sc values (6,'吕布','一年二班',67);
insert into sc values (7,'赵云','一年二班',90);
insert into sc values (8,'典韦','一年二班',89);
insert into sc values (9,'关羽','一年二班',70);
insert into sc values (10,'马超','一年二班',98);

select *
from sc t
where (select count(1)+1 from sc where class=t.class and score>t.score)<= 3

SELECT *
FROM sc a
WHERE 3 > (
SELECT COUNT( * )
FROM sc
WHERE class = a.class
AND score > a.score )
ORDER BY a.class , a.score DESC

select * from t_category where description<>'325'

select * from t_category where description='325'

select sum(user_id) as ids from t_category where description='aaaaa'

       select 5=5;  -- trueselect cast(5 as char(20))=5;  -- trueselect cast(55 as char(20))=5;  -- falseselect cast('5,2' as char(20))=5;  -- trueselect '5,2,4'=5; -- trueselect 5='5,4';   -- trueselect 5='5^4';  -- trueselect 5='5|4';  -- trueselect 5='5.4';  -- false

        select find_in_set('b','a,b,c,d,e,f');   -- 2select find_in_set('d','a,b,c,d,e,f');   -- 4