mysql子查询

-- 子查询(实质：where中嵌套查询语句)
 -- 查询语文五十分以下的学生姓名	
SELECT  sid,sname from student WHERE sid in (SELECT sid from sc WHERE score <50 and cid=1) 



CREATE TABLE `category`( `categoryid` INT(3) NOT NULL COMMENT 'id', `pid` INT(3) NOT NULL COMMENT '父id 没有父则为1', `categoryname` VARCHAR(10) NOT NULL COMMENT '种类名字', PRIMARY KEY (`categoryid`) ) ENGINE=INNODB CHARSET=utf8 COLLATE=utf8_general_ci; 

INSERT INTO `category` (`categoryid`, `pid`, `categoryname`) VALUES ('2', '1', '信息技术');
insert into `CATEGOrY` (`categoryid`, `pid`, `categoryname`) values ('3', '1', '软件开发');
insert into `category` (`categoryid`, `PId`, `categoryname`) values ('5', '1', '美术设计');
insert iNTO `category` (`categoryid`, `pid`, `categorynamE`) VAlUES ('4', '3', '数据库'); 
insert into `category` (`CATEgoryid`, `pid`, `categoryname`) values ('8', '2', '办公信息');
insert into `category` (`categoryid`, `pid`, `CAtegoryname`) values ('6', '3', 'web开发'); 
inserT INTO `category` (`categoryid`, `pid`, `categoryname`) valueS ('7', '5', 'ps技术');
-- 自连接查询
SELECT c1.categoryname,c2.categoryname FROM category c1,category c2 where c1.pid=c2.categoryid;