面试题(数据库)

1>为管理业务培训信息，建立3个表：

S(S#,SN,SD,SA)S#,SN,SD,SA分别代表学号，学员姓名，所属单位，学员年龄
 
C(C#,CN)C#,CN分别代表课程编号，课程名称

SC(S#,C#,G) S#,C#,G分别代表学号，所选的课程编号，学习成绩

（1）使用标准SQL嵌套语句查询选修课程名称为’税收基础’的学员学号和姓名?

答案：select s# ,sn from s where S# in(select S# from c,sc where c.c#=sc.c# and cn=’税收基础’)

(2) 使用标准SQL嵌套语句查询选修课程编号为’C2’的学员姓名和所属单位?

答：select sn,sd from s,sc where s.s#=sc.s# and sc.c#=’c2’

(3) 使用标准SQL嵌套语句查询不选修课程编号为’C5’的学员姓名和所属单位?

答：select sn,sd from s where s# not in(select s# from sc where c#=’c5’)

(4)查询选修了课程的学员人数

答：select 学员人数=count(distinct s#) from sc

(5) 查询选修课程超过5门的学员学号和所属单位?

答：select sn,sd from s where s# in(select s# from sc group by s# having count(distinct c#)>5)

2>
 SQL求出小于45岁的各个老师所带的大于12岁的学生人数
                  数据库中有3个表 teacher 表，student表，tea_stu关系表。
                  teacher 表 teaID name age
                  student 表 stuID name age
                  teacher_student表 teaID stuID
                  要求用一条sql查询出这样的结果
                  1.显示的字段要有老师name, age 每个老师所带的学生人数
                  2 只列出老师age为40以下，学生age为12以上的记录
                  预备知识：
                  1.sql语句是对每一条记录依次处理，条件为真则执行动作（select,insert,delete,update）
                  2.只要是迪卡尔积，就会产生"垃圾"信息，所以，只要迪卡尔积了，我们首先就要想到清除"垃圾"信息
                  实验准备：
                  drop table if exists tea_stu;
                  drop table if exists teacher;
                  drop table if exists student;
                  create table teacher(teaID int primary key,name
                  varchar(50),age int);
                  create table student(stuID int primary key,name
                  varchar(50),age int);
                  create table tea_stu(teaID int references teacher(teaID),stuID
                  int references student(stuID));
                  insert into teacher values(1,'zxx',45), (2,'lhm',25) ,
                  (3,'wzg',26) , (4,'tg',27);
                  insert into student values(1,'wy',11), (2,'dh',25) ,
                  (3,'ysq',26) , (4,'mxc',27);
                  insert into tea_stu values(1,1), (1,2), (1,3);
                  insert into tea_stu values(2,2), (2,3), (2,4);
                  insert into tea_stu values(3,3), (3,4), (3,1);
                  insert into tea_stu values(4,4), (4,1), (4,2) , (4,3);
                  结果：2?3,3?2,4?3
                  解题思路：
                  1要会统计分组信息，统计信息放在中间表中：
                  select teaid,count(*) from tea_stu group by teaid;
                  2接着其实应该是筛除掉小于12岁的学生，然后再进行统计，中间表必须与student关联才能得到12岁以下学生和把该学生记录从中间表中剔除，代码是：
                  select tea_stu.teaid,count(*) total from student,tea_stu
                  where student.stuid=tea_stu.stuid and student.age>12 group by
                  tea_stu.teaid
                  3.接着把上面的结果做成虚表与teacher进行关联，并筛除大于45的老师
                  select teacher.teaid,teacher.name,total from teacher ,(select
                  tea_stu.tea
                  id,count(*) total from student,tea_stu where
                  student.stuid=tea_stu.stuid and stu
                  dent.age>12 group by tea_stu.teaid) as tea_stu2 where
                  teacher.teaid=tea_stu2.tea
                  id and teacher.age<45;