Database interim (II)

Student information is stored in the educational administration system , Including student ID 、 full name 、 major ; Course information , Include course number 、 Course name ,

credits , Students' course selection information , Include , Student number 、 Course no. 、 achievement .

1) Write course information 、 Database mode of course selection information ;

Student ( Student number , full name , major ); Course ( Course no. , Course name , credits ); Course selection ( Student number , Course no. , achievement );

2) Write out the command of creating course information ;

Create table Course ( Course no. char(6), Course name varchar(30), credits decimal(2,1), Primary key ( Course no. ) ); create table stu( sno char(12), name varchar(120) not null, major varchar(30) not null, Primary key (sno) ); Coment on table stu is ‘ Student information sheet ’; Coment on column stu.sno is ‘ Student number ’; ALTER TABLE stu COMMENT=' Student information sheet '; ALTER TABLE stu MODIFY sno CHAR(6) COMMENT ' Student number '; Create table course( cno char(7), cname varchar(30) not null, credit decimal(2,1) not null, Primary key (cno) ); Create table sc( sno char(12), cno char(7), score decimal(4,1), Primary key (sno,cno) );

3) Write the relational algebraic expression of the following query ：

        a) The result is less than 60 The student number of the student ;

Select sno From sc Where score < 60;

        b) The result is less than 60 The student number of the student 、 full name ;

Select stu.sno, name From stu, sc Where stu.sno = sc.sno And score < 60;

        c) Find out the student ID of students who have never taken elective courses ;

Select sno From stu Where sno not in (Select sno From sc);

        d) Find out the student number of the student with the lowest score ;

Select sno From sc Where score = (select min(score) from sc);

        e) At least 2 The number of courses selected by students ;

Select distinct sc.cno From sc, sc s1 Where sc.cno = s1.cno and sc.sno <> s1.sno;

         f) Find out the student ID of students who have taken all courses

Select distinct sno From sc a Where not exists (         select * From course b Where not exists (                 select * from sc where sno = a.sno  and cno = b.cno));

        g) Please at least choose C001 and C003 Students of two courses learn Number

select  distinct  sno  from  sc  A  where  not  exists(        select  *  from   course B where  cno   in ('C002','C003')  and   not  exists(               select * from  sc C where   A.sno=C.sno   and   B.cno=C.cno));

R(X,Y)÷S(Y,Z) The operation of is in structured language SQL The statement can be expressed in the following form :

select distinct R.X from R R1

where not exists (select S.Y

                              from S

                              where not exists (select * from R R2

                                                            where R2.X = R1.X and R2.Y = S.Y));