大牛请接题,该如何处理

drop table stu_cour;drop table student;drop table course;create table student(sid char(10) constraint student_sid_pk primary key, --学号sname varchar(20) not null --姓名);create table course(cid char(10) constraint course_cid_pk primary key, --学号cname varchar(20) not null --班级名);create table stu_cour(sid char(10) constraint stu_cour_sid_fk references student(sid),cid char(10) constraint stu_cour_cid_fk references course(cid),score number(4,1) constraint stu_cour_score_ck check (score between 0 and 100),constraint stu_cour_sid_cid_pk primary key (sid,cid));drop sequence s1;create sequence s1start with 1001increment by 1;drop sequence s2;create sequence s2start with 1001increment by 1;insert into student(sid, sname) values ('s'||s1.nextval,'a');insert into student(sid, sname) values ('s'||s1.nextval,'b');insert into student(sid, sname) values ('s'||s1.nextval,'c');insert into course(cid, cname) values ('c'||s2.nextval, 'c1');insert into course(cid, cname) values ('c'||s2.nextval, 'c2');insert into course(cid, cname) values ('c'||s2.nextval, 'c3');insert into stu_cour(sid, cid, score) values ('s1001', 'c1001', 90);insert into stu_cour(sid, cid, score) values ('s1001', 'c1002', 80);insert into stu_cour(sid, cid, score) values ('s1001', 'c1003', 80);insert into stu_cour(sid, cid, score) values ('s1002', 'c1001', 90);insert into stu_cour(sid, cid, score) values ('s1002', 'c1002', 80);insert into stu_cour(sid, cid, score) values ('s1003', 'c1001', 90);insert into stu_cour(sid, cid, score) values ('s1003', 'c1002', 80);insert into stu_cour(sid, cid, score) values ('s1003', 'c1003', 80);commit;

select c.cidfrom course c left join stu_cour sc on c.cid = sc.cid and sc.sid = ('s1002') --对where sc.sid is null and sc.cid is null;这样可以实现select c.cidfrom course c left join stu_cour sc on c.cid = sc.cid and (sc.sid = (select sid                    from student                    where sname = 'b')) -- 错了where sc.sid is null and sc.cid is null;但将其换成子查询为啥就不可以了？子查询的结果是's1002'啊各个子句的执行顺序不是 from --> and --> on --> where --> select 么？

--应该改为in就可以了，因你的sc.sid为一条记录，不能等于一个记录集，只能是存在于(in)select c.cidfrom course c left join stu_cour sc on c.cid = sc.cid and (sc.sid in (select sid                    from student                    where sname = 'b')) where sc.sid is null and sc.cid is null;
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SQL code



drop table stu_cour;
drop table student;
drop table course;

create table student(
sid char(10) constraint student_sid_pk primary key, --学号
sname varchar(20) not null --姓名
);

create tabl……

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上面写了这么多建表语句  直接上下面俩sql就可以了 

唯一关系才返回一个值  多条记录 in返回数据集