SQL 增删改查（学生、课程、选课表为例）

初始表结构和内容

包含Student、Course和SC三个表（分别是学生信息表，课程信息表和选课成绩表）

1.1 Student 表结构

1.2 Course 表结构

1.3 SC表结构

2.1 Student内容

2.2 Course内容

SELECT * FROM Course ORDER BY cast(Cno as int);

2.3 SC内容

插入数据

INSERT INTO Student VALUES('201212126','WANGHUA','MALE',18,'CS');

删除数据

DELETE FROM Course WHERE Cno = '11';

更新数据

UPDATE Course SET Cno = '8' WHERE Cno = '11';

查找数据

1.单表查询

SELECT Sno, Sname FROM Student;

SELECT Sname, 2021-Sage FROM Student;

SELECT Sname NAME,
'Year of Birth:' BIRTH, 
2021-Sage BIRTHDAY,
LOWER(Sdept) DEPARTMENT 
FROM Student;

SELECT Sno FROM SC;

SELECT DISTINCT Sno FROM SC;

SELECT Sname FROM Student WHERE Sdept = 'CS';

SELECT Sname, Sage FROM Student WHERE Sage<20;

SELECT Sname, Sdept, Sage FROM Student WHERE Sage (NOT) BETWEEN 19 AND 20;

SELECT Sname, Sdept, Sage FROM Student WHERE Sage BETWEEN 19 AND 20;

(NOT)

SELECT Sname, Ssex FROM Student WHERE Sdept IN('CS','IS');

模糊查询
通配符: _ %

SELECT * FROM Student WHERE Sno LIKE '201212121';

SELECT * FROM Course WHERE Cno = '2';

SELECT * FROM Course WHERE Cno LIKE '2';

SELECT * FROM Course WHERE Cno LIKE '2%';

SELECT Sname FROM Student WHERE Sname LIKE 'LIU%';

SELECT Sname FROM Student WHERE Sname LIKE '___CHEN%';

INSERT INTO Course VALUES('9','DBADESIGN','',4);

SELECT * FROM Course WHERE Cname LIKE 'DB_DESIGN%';

SELECT * FROM Course WHERE Cname LIKE 'DB\_DESIGN%' ESCAPE'\';

聚集函数

学生总人数

SELECT COUNT(*) FROM Student;

选课1的平均成绩

SELECT AVG(Grade) FROM SC WHERE Cno = '1';

学生201212121选修课程的总学分数

SELECT SUM(Ccredit) FROM SC,Course 
WHERE Sno = '201212121' AND SC.Cno = Course.Cno;

GROUP BY

各课号及相应的选课人数

SELECT Cno, COUNT(Sno) FROM SC GROUP BY Cno;

链接查询

每个学生及其选修课程的情况

SELECT Student.*, SC.* FROM Student,SC WHERE  Student.Sno = SC.Sno;

自然连接

SELECT Student.Sno, Sname, Ssex, Sage, Sdept, Cno, Grade
FROM Student, SC WHERE Student.Sno = SC.Sno;

选修2号课程且成绩在90分以下的学生学号和姓名

SELECT Student.Sno, Sname FROM Student, SC 
WHERE Student.Sno = SC.Sno AND SC.Cno = '2' AND SC.Grade<=90;

自身连接

查询每一门课程的先修课（对比）

 SELECT FIRST.Cno,SECOND.Cpno FROM Course FIRST, Course SECOND 
 WHERE FIRST.Cpno = SECOND.Cno;

SELECT Cno,Cpno FROM Course WHERE Cpno = Cno;

SELECT FIRST.Cno,FIRST.Cpno FROM Course FIRST,Course 
WHERE FIRST.Cpno = FIRST.Cno;

外连接

SELECT Student.Sno, Sname, Sname, Ssex, Sage, Sdept,Cno,Grade 
FROM Student LEFT OUTER JOIN SC ON (Student.Sno = SC.Sno); 

多表连接

查询每个学生的学号、姓名、选修的课及成绩

SELECT Student.Sno, Sname, Cname, Grade FROM Student,SC,Course
 WHERE Student.Sno = SC.Sno AND SC.Cno = Course.Cno;

嵌套查询

选取了2号课程的学生姓名

SELECT Sname FROM Student WHERE Sno IN 
(SELECT Sno FROM SC WHERE Cno = '2');

每个学生超过他自己选修课程平均成绩的课程号

SELECT Sno, Cno FROM SC x 
WHERE Grade >=(SELECT AVG(Grade) FROM SC y WHERE y.Sno = x.Sno);

集合查询

计算机科学系的学生以及年龄不大于19岁的学生

SELECT * FROM Student WHERE Sdept = 'CS' 
UNION SELECT * FROM Student WHERE Sage<=19;

计算机科学系的学生与年龄不大于19岁的交集

SELECT * FROM Student WHERE Sdept = 'CS' 
INTERSECT SELECT * FROM Student WHERE Sage<=19;

计算机科学与技术系的学生与年龄不大于19岁的学生的差集

SELECT * FROM Student WHERE Sdept = 'CS' 
EXCEPT SELECT * FROM Student WHERE Sage<=19;

基于派生表的查询

找出每个学生超过他自己选修课程平均成绩的课程号

SELECT Sno, Cno 
FROM SC, (SELECT Sno, Avg(Grade) FROM SC GROUP BY Sno) 
AS Avg_sc(avg_sno,avg_grade)
WHERE SC.Sno = Avg_sc.avg_sno AND SC.Grade >= Avg_sc.avg_grade;