select * from dept,employee where exists(select did from employee where age>21);
create view view_sales as select first_half+latter_half from sales
select p1.* from employee p1 join employee p2 on  p1.did =p2.did where p2.name="王红"；
select * from send where datediff(dd,sendtime,getdate())=0;
CREATE INDEX index_subject 一>  ON content(subject(3)ASC);
select  * from score group by 学号 having sum(分数)>180;

CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost`  VIEW `v_hpkcb` AS SELECT
    hpkcb.kcid, 
    hpkcb.kcsl, 
    hpmcb.hpmc, 
    hpmcb.jldw
FROM
    hpkcb
    INNER JOIN
    hpmcb
    ON 
        hpkcb.kcid = hpmcb.Hpid
————————————————