数据库概论_3标准化的SQL语言_ 数据定义

Student:

Course:

 SC:

1.数据库的定义、删除

1.1数据库的定义（创建）

 CREATE DATABASE student; 

1.2数据库的删除

 DROP DATABASE student; 

2.表的定义、修改与删除

2.1表的定义

建表语句：

CREATE TABLE student(
Sno VARCHAR(255) PRIMARY KEY,
Sname VARCHAR(255),
Ssex CHAR(4),
Sage SMALLINT,
Sdept CHAR(4)
);

添加数据：

INSERT INTO student(Sno,Sname,Ssex,Sage,Sdept) VALUES ("20170707001","张一","男",20,"CS");
INSERT INTO student VALUES ("20170707002","张二","男",21,"CS");
INSERT INTO student(Sno,Sname,Ssex,Sage,Sdept) VALUES ("20170707003","张三","女",22,"MA");
INSERT INTO student(Sno,Sname,Ssex,Sage,Sdept) VALUES ("20170707004","张四","女",18,"IS");

2.2表的修改

 2.2.1向student表中添加“入学时间”列，数据类型为日期型

 SQL语句：

ALTER TABLE student ADD s_entrance DATE;

结果：

2.2.2将年龄的数据类型由字符型改为整型

mysql>>

ALTER TABLE student MODIFY COLUMN Sage INT;

 

2.2.3修改表名

ALTER TABLE studentt RENAME TO course;

2.3表的删除

DROP TABLE course;

3.数据查询

3.1单表查询

3.1.1查所有

-- 方式一
SELECT * from student;
-- 方式二
SELECT Sno,Sname,Ssex,Sage,Sdept,S_entrance from student;

3.1.2选择查询表中某些列

SELECT Sno,Sname,Sdept FROM student;

3.1.3查询经过计算的值并起个别名birthday

SELECT Sname,2014-Sage as birthday FROM student;

3.1.4 查询所在系（用小写字母表示系名）

SELECT Sname,LOWER(Sdept) FROM student;

3.1.5去除结果中重复的行

SELECT DISTINCT s_entrance FROM student;

3.1.6查询满足条件的元组

1)年龄小于25岁的

SELECT * FROM student WHERE Sage<25;

2）Sno等于20170707001的

-- Sno值可以加""，也可以不加""
SELECT * FROM student WHERE Sno="20170707001";

3)年龄在20到30之间的

SELECT * FROM student WHERE Sage BETWEEN 20 AND 30;
-- 等价于
SELECT * FROM student WHERE Sage >= 20 AND Sage <=30;

4）名字中含有“一”的

SELECT * FROM student WHERE Sname LIKE "_一";

5）名字中不含“一”的

SELECT * FROM student WHERE Sname NOT LIKE "_一";

6)按学号降序排列

-- 默认升序（ASC）

SELECT * FROM student ORDER BY Sno DESC;

7）聚集函数

-- 统计元组个数
SELECT COUNT(*) FROM student;
-- 统计一列中的个数(空值不算)
SELECT COUNT(DISTINCT s_entrance) FROM student;
-- 计算一列值的总合（必须为int）
SELECT SUM(Sage) FROM student;
-- 计算一列值的平均值
SELECT AVG(DISTINCT Sage) FROM student;
-- 找一个值的最大值
SELECT MAX(Sage) FROM student;
-- 找一列值的最小值
SELECT MIN(Sage) FROM student;

8)GROUP BY：分组语句

SELECT Sname,SUM(Sage) FROM student GROUP BY Sname;

-- GROUP BY 与WHERE语句不能共用
SELECT Sname,SUM(Sage) FROM student GROUP BY Sname HAVING SUM(Sage)>=50;

3.2连接查询

3.2.1等值与非等值连接查询

等值连接与非等值连接：当连接运算符为“=”时，称为等值连接。使用其它运算符称为非等值连接。

 例：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;

把目标列中重复的属性列去掉则为自然连接。

3.2.2自身连接

例：查询每一门课的间接先修课

SELECT f.Cno,s.Cpno
FROM Course f,Course s
WHERE f.Cpno=s.Cno;

3.2.3外连接

分为：左外连接，右外连接，内连接

left join,right join,inner join

如：

SELECT * FROM Course f LEFT JOIN Course s ON f.Cpno=s.Cno;
SELECT * FROM Course f RIGHT JOIN Course s ON f.Cpno=s.Cno;
SELECT * FROM Course f INNER JOIN Course s ON f.Cpno=s.Cno;

3.2.4多表连接

两个表以上的操作称为多表连接

例：查询每个学生的学号，姓名，选修的课程名及成绩

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

3.3嵌套查询

-- 子查询：嵌套查询
-- '='子查询结果只有一个
SELECT * FROM student WHERE banji_id=(SELECT id FROM banji WHERE classname='Java1812');
-- 子查询结果不只一个
SELECT * FROM student WHERE banji_id IN (SELECT id FROM banji WHERE classname IN 'Java1812','Java1807'));