SQL大神指教指教，谢谢！
表一:XS
Sno Sname Sex birthday Pid class department (表一属性)

表二:CJ
Sno Courseno Score (表二属性)

表三：KC
Courseno Coursename credit (表三属性)

要求："用相关查询语句"查询选修课程号为”xx“ 的学生的相关信息

select XS.Sno, Sname,Sex, birthday, Pid, class, department,
KC.Courseno, Coursename, credit, Score
from XS
left join CJ on CJ.Sno = XS.Sno
left join KC on KC.Courseno = CJ.Courseno

 select x.Sno,x.Sname,x.Sex,x.birthday from XS x where x.Sno=(select   c.Sno  from CJ c where c.Courseno =  (select k.Courseno from Coursename = "xx"))

select XS.Sno,XS.Sname,XS.Sex,XS.birthday,KC. Coursename,CJ.Score
where XS.Sno=CJ.Sno and CJ.Courseno=KC.Courseno

想要显示哪个属性，在select后加就好了

 SELECT x.Sname from XS x where x.Sno IN
(SELECT c.Sno from CJ c where c.Courseno = 
(SELECT k.Courseno from KC k where k.Coursename = "xx"));

根据你的需要可以使用left join 或者 inner join

我好像忘了加 from table啦 0.0