SQL简单查询——学生课程表

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一、学习目标

1、掌握$SELECT$语句的语法格式，各句子的使用；
2、掌握别名的概念、掌握$DISTINCT$、$ALL$的使用;
3、掌握空值的概念及查询判断条件的表达，了解查询时系统对$NULL$值的处理；
4、掌握使用$GROUPBY$子句、$HAVING$子句、$ORDERBY$子句对查询结果进行排序和分组；
5、掌握$SUM$、$AVG$、$COUNT$、$MAX$、$MIN$等聚合函数的用法。

二、数表介绍

本次是数据库中最典型的学生课程表，如下如所示：

• tb_sc
  
• tb_student
  
• tb_course
  

三、SQL简单查询

use tb;#首先要选择数据库
SELECT * FROM tb_student;
SELECT Sname 姓名,2020-Sage 出生年份, LOWER(Sdept) 所属系院 FROM tb_student;
SELECT Sno FROM tb_sc;
SELECT Sname,Sage FROM tb_student WHERE Sage=19;
SELECT Sname,Sdept,Sage FROM tb_student WHERE Sage NOT IN (20,23);
SELECT Sname ,Ssex FROM tb_student WHERE Sdept NOT IN ('IS','MA','CS');
SELECT * FROM tb_student st ,tb_sc sc WHERE st.Sno='201812119'AND sc.Sno='201812119';
SELECT Sname FROM tb_student WHERE Sname LIKE '刘_礼';
SELECT Sname FROM tb_student WHERE Sname  not LIKE '刘%';
SELECT * FROM tb_course WHERE  Cname LIKE 'DB#_%i__' ESCAPE '#';
SELECT Sno,Cno FROM tb_sc WHERE Grade IS NOT NULL;
SELECT Sname, Sage, Sdept FROM tb_student WHERE Sage<20 AND Sdept IN ('CS');
SELECT Cno,Grade FROM tb_sc WHERE Cno=3 ORDER BY Grade DESC;
SELECT * FROM tb_student ORDER BY Sdept ASC,Sage DESC;# 年龄降序
SELECT Sname FROM tb_student ORDER BY Sage DESC LIMIT 3;
SELECT COUNT(DISTINCT Sno) 总人数 FROM tb_sc;#计算总人数 去除重复的
SELECT AVG(Grade) 平均值 ,MAX(Grade) 最大值,CNO FROM tb_sc WHERE Cno=2;
SELECT  Cno,GROUP_CONCAT(Grade) AS 成绩 ,COUNT(Grade)FROM tb_sc GROUP BY Cno;#根据Cno分类
SELECT Cno,COUNT(Cno)FROM tb_sc GROUP BY Cno;
SELECT Sno 学生学号,GROUP_CONCAT(Cno) 课程编码 FROM tb_sc GROUP BY Sno HAVING count(Cno)>2; 
SELECT Sno 学号,GROUP_CONCAT(Grade) score FROM tb_sc WHERE Grade>80 GROUP BY Sno HAVING COUNT(Grade)>2 ;
SELECT SUM(Grade) 总分 FROM tb_sc WHERE Sno='201215121';

其中有两个sql语句有图展示：

• SELECT Cno,GROUP_CONCAT(Grade) AS 成绩 ,COUNT(Grade)FROM tb_sc GROUP BY Cno;#根据Cno分类
  

• SELECT Sno 学生学号,GROUP_CONCAT(Cno) 课程编码 FROM tb_sc GROUP BY Sno HAVING count(Cno)>2;
  

四、SQL复杂查询

1. 查询每一门课的间接先修课（即先修课的先修课）

SELECT a.Cno,b.Cpno FROM tb_course a,tb_course b WHERE a.Cpno=b.Cno

2. 查询每个学生选修课程的情况

SELECT * FROM tb_student,tb_sc WHERE tb_student.Sno=tb_sc.Sno;

3. 查询每个学生及其选修课程的情况包括没有选修课程的学生（用外连接）

SELECT tb_student.Sno,Sname,Ssex,Sage,Sdept,Cno,Grade
FROM tb_student LEFT JOIN tb_sc USING(Sno) ;

4. 查询选修2号课程且成绩在90分以上的所有学生的学号、姓名

SELECT tb_student.Sno,Sname FROM tb_student,tb_sc WHERE tb_student.Sno=tb_sc.Sno AND tb_sc.Cno='2' AND tb_sc.Grade>90;

5. 查询学生201215121选修课程的总学分

SELECT SUM(Grade) 总分 FROM tb_sc WHERE Sno='201215121';

6. 查询每个学生选修课程的总学分

SELECT Sno,GROUP_CONCAT(Grade) 分数 ,SUM(Grade) 总和 FROM tb_sc GROUP BY Sno;

7. 查询每个学生的学号、姓名、选修的课程名及成绩

SELECT tb_student.Sno,Sname,Cname,Grade FROM tb_sc,tb_course,tb_student WHERE tb_student.Sno=tb_sc.Sno AND tb_sc.Cno=tb_course.Cno;

8. 查询与“刘晨礼”在同一个系学习的学生（分别用嵌套查询和连接查询）

-- 嵌套查询
SELECT Sno,Sname,Sdept FROM tb_student WHERE Sdept = (SELECT Sdept FROM tb_student WHERE Sname='刘晨礼');
-- 自身连接查询
SELECT a.Sno,a.Sname,a.Sdept FROM tb_student a,tb_student b WHERE a.Sdept=b.Sdept AND b.Sname='刘晨礼';

9. 查询选修了课程名为“信息系统”的学生学号和姓名

SELECT tb_student.Sno,Sname FROM tb_sc,tb_course,tb_student WHERE tb_student.Sno=tb_sc.Sno AND tb_sc.Cno=tb_course.Cno AND tb_course.Cname='信息系统';