mysql:day3--java访问数据库、CASE...WHEN、无关/相关子查询、表与表之间的关系

java访问数据库

分为四步：

1、加载连接器

高级版本的mysql可以省略这一步

Class.forName("com.mysql.jdbc.Driver");

2、建立连接

我们最好在建立连接的时候指定编码，这样可以保证执行的时候不会因此乱码。当然也可以采用默认编码方式连接，不过不推荐。

//String url = "jdbc:mysql://127.0.0.1:3306/aa";
String url = "jdbc:mysql://127.0.0.1:3306/aa?useUnicode=true&characterEncoding=UTF-8";
Connection con = DriverManager.getConnection(url,"root","");

3、获取语句对象

Statement st = con.createStatement();

4、对数据库进行操作

对于sql语句都可以用

st.exectute(sql);

来执行，只不过这个返回的是一个boolean值，表示是否执行成功。我们在查询的时候，希望返回的是一些数据，此时，用这种方式显然不合理。所以我们对于查询采用

st.executeQuery(sql)

来做。这个函数返回的是一个ResultSet集合对象，因为第一个是表头，我们不需要，所以我们直接在while中用rs.next()来做操作。而且要注意的是，

rs.getString(1);

第一列的序号不是0，而是1；

//4 对数据库进行操作
//增删改用 st.exectute(sql);
//增     String sql = "insert into sstud values('1015','张三',20,'武汉');";
//删     String sql = "delete from sstud where sno='1010';";
//改     String sql = "update sstud set saddress='城院' where sno='1009';";
//st.execute(sql);
//查
String sql = "select * from sstud;";
ResultSet rs = st.executeQuery(sql);
while(rs.next()){
String sno = rs.getString(1);//第一列的序号是1.不是0;
String sname = rs.getString("sname");
int age = rs.getInt("age");
String saddress = rs.getString(4);
System.out.println(sno+","+sname+","+age+","+saddress);
}

数据库编码

查看当前数据库编码

SHOW VARIABLES WHERE variable_name LIKE 'character%';

解决中文乱码：

要求：保证如下三个编码一致。

1、character_set_client

2、character_set_connection

3、character_set_results

设置编码

SET character_set_results= gbk;

控制流程函数 CASE…WHEN

实际语法是：CASE…WHEN…THEN…END

例子：当sex值为1时，显示男 ，当sex值为0时，显示女；

SELECT sno,sname,age,saddress,(CASE sex WHEN '0' THEN '女' WHEN '1' THEN '男' END ) AS 性别 FROM sstud;

无关/相关子查询

无关子查询

例子：查询哪些人有同龄人

SELECT * FROM sstud GROUP BY age HAVING COUNT(age)>=2;

相关子查询

例子：查询哪些人有同龄人并且性别为男

SELECT * FROM sstud GROUP BY age HAVING COUNT(age)>=2 AND sex='1';

表与表之间的关系

1一对一

需要两个表，做项目时通常只建一个物理表。如果要实现一对一的查询，建立两个视图。

/*1.1建立物理表 初始化数据*/
CREATE TABLE person(
id INT,
NAME VARCHAR(10),
sex CHAR(1),
wife INT,
husband INT
);
INSERT INTO person VALUES(1,'小花','0',0,3);
INSERT INTO person VALUES(2,'玉芬','0',0,4);
INSERT INTO person VALUES(3,'张三','1',1,0);
INSERT INTO person VALUES(4,'李四','1',2,0);
INSERT INTO person VALUES(5,'王五','1',0,0);
/*1.2建立两个视图*/
CREATE VIEW women AS SELECT * FROM person WHERE sex='0';
CREATE VIEW men AS SELECT * FROM person WHERE sex='1';
/*1.3查询*/
/*老版本的1:1查询*/
SELECT women.NAME AS 妻子, men.NAME AS 丈夫 FROM women , men WHERE  women.husband=men.id;
/*利用连接进行1:1查询*/
/*右连接*/
SELECT women.NAME AS 妻子, men.NAME AS 丈夫 FROM women RIGHT JOIN men ON women.husband=men.id;
/*左连接*/
SELECT women.NAME AS 妻子, men.NAME AS 丈夫 FROM women LEFT JOIN men ON women.husband=men.id;

2一对多

建立两张表，一张独立表，另一张表用外键连接此表。

/*2一对多*/
/*建立表格 初始化*/
CREATE TABLE person2(
id VARCHAR(32) PRIMARY KEY,
NAME VARCHAR(30),
sex CHAR(1),
age INT
);
INSERT INTO person2 VALUES('P1001','小花','0',25);
INSERT INTO person2 VALUES('P1002','张三','1',22);
INSERT INTO person2 VALUES('P1003','Jack','1',24);
INSERT INTO person2 VALUES('P1004','Rose','0',25);
CREATE TABLE car(
id VARCHAR(32) PRIMARY KEY,
NAME VARCHAR(30),
price NUMERIC(10,2),
pid VARCHAR(32),
/*为字段pid定义一个外键约束(来自person2表的字段id)*/
CONSTRAINT car_fk FOREIGN KEY(pid) REFERENCES person2(id)
);
INSERT INTO car VALUES('C001','BMW',80.5,'P1001');
INSERT INTO car VALUES('C002','Benz',100,'P1001');
INSERT INTO car VALUES('C003','BMW',120.05,'P1001');
INSERT INTO car VALUES('C004','Benz',88.5,'P1002');
INSERT INTO car VALUES('C005','QQ',8.5,'P1002');
INSERT INTO car VALUES('C006','BIKE',0.5,'P1003');

/*这时候不能进行删除 因为主键这时是car表格的外键依赖*/
DELETE FROM person2 WHERE(id='P1001');

3多对多

/*  3、多对多
数据库设计分析
※案例：一个人可以选择多门课程，一门课程又可以被很多人选择。
*/
/*3.1 学生表--实体*/
CREATE  TABLE student(
id VARCHAR(32) PRIMARY KEY,
NAME VARCHAR(30),
age INT
);
/*3.2 课程表--实体*/
CREATE TABLE ject(
id VARCHAR(32) PRIMARY KEY,
NAME VARCHAR(30)
);
/*3.3 关系表--关系*/
CREATE TABLE sj(
studid VARCHAR(32),
subjectid VARCHAR(32)
);
/*3.4 单独添加约束(注意顺序: 要先添加联合主键，再添加外键)*/
/*创建联合主键*/
ALTER TABLE sj ADD CONSTRAINT pk_sj PRIMARY KEY(studid,subjectid);
/*创建两个外键*/
ALTER TABLE sj ADD CONSTRAINT pk_stud FOREIGN  KEY(studid) REFERENCES student(id);
ALTER TABLE sj ADD CONSTRAINT pk_subj FOREIGN KEY(subjectid) REFERENCES ject(id);

/*3.5初始化数据*/
INSERT INTO student VALUES('P001','小花',25);
INSERT INTO student VALUES('P002','Jack',23);
INSERT INTO student VALUES('P003','Tom',24);
INSERT INTO student VALUES('P004','张三',24);
INSERT INTO student VALUES('P005','赵子龙',26);

INSERT INTO ject VALUES('S001','Java');
INSERT INTO ject VALUES('S002','JavaEE');
INSERT INTO ject VALUES('S003','XML');
INSERT INTO ject VALUES('S004','数据库');
INSERT INTO ject VALUES('S005','JQuery');

INSERT INTO sj VALUES('P001','S001');
INSERT INTO sj VALUES('P001','S003');
INSERT INTO sj VALUES('P002','S001');
INSERT INTO sj VALUES('P002','S002');
INSERT INTO sj VALUES('P002','S003');
INSERT INTO sj VALUES('P003','S001');
INSERT INTO sj VALUES('P004','S002');
INSERT INTO sj VALUES('P004','S003');

/*3.6 数据库操作*/
/*关联: 内连接、左关联、右关联*/
/*要求：查询哪些人选了哪些课*/
SELECT student.NAME AS 学生姓名,ject.NAME AS 课程名称 FROM student INNER JOIN sj ON student.id=sj.studid INNER JOIN ject ON sj.subjectid = ject.id;
/*要求：查询哪些人没有选课*/
SELECT student.NAME AS 学生姓名   FROM student LEFT JOIN sj ON student.id=sj.studid LEFT JOIN ject ON sj.subjectid=ject.id WHERE ject.NAME IS NULL;
/*要求：查询哪些课程没有人选*/
SELECT ject.id AS 课程编号, ject.NAME AS 课程名称 FROM student RIGHT JOIN sj ON student.id=sj.studid RIGHT JOIN ject ON sj.subjectid=ject.id WHERE student.name IS NULL;
/*注意：在左右关联中没有绝对，只是看你怎么连表*/

mysql第三天课堂记录

/*查询当前数据库编码*/
SHOW VARIABLES WHERE variable_name LIKE 'character%';
/*解决中文乱码*/
/*要求：character_set_client、character_set_connection、character_set_results保持编码一致*/
/*设置编码*/
SET character_set_results= gbk;
SELECT * FROM sstud;
SET character_set_results= utf8;

/*控制流程函数 CASE...WHEN...THEN...END*/
/*增加一列*/
ALTER TABLE sstud ADD COLUMN sex CHAR(1);
UPDATE sstud SET sex=1 WHERE sex IS NULL;
SELECT sno,sname,age,saddress,(CASE sex WHEN '0' THEN '女' WHEN '1' THEN '男' END ) AS 性别 FROM sstud;

/*无关子查询*/
/*查那些人有同龄人*/
SELECT * FROM sstud GROUP BY age HAVING COUNT(age)>=2;
/*查那些人有同龄人且性别为男*/
SELECT * FROM sstud GROUP BY age HAVING COUNT(age)>=2 AND sex='1';

/*表与表之间的关系*/
/*1 一对一*/
/*1.1建立物理表 初始化数据*/
CREATE TABLE person(
id INT,
NAME VARCHAR(10),
sex CHAR(1),
wife INT,
husband INT
);
INSERT INTO person VALUES(1,'小花','0',0,3);
INSERT INTO person VALUES(2,'玉芬','0',0,4);
INSERT INTO person VALUES(3,'张三','1',1,0);
INSERT INTO person VALUES(4,'李四','1',2,0);
INSERT INTO person VALUES(5,'王五','1',0,0);
/*1.2建立两个视图*/
CREATE VIEW women AS SELECT * FROM person WHERE sex='0';
CREATE VIEW men AS SELECT * FROM person WHERE sex='1';
/*1.3查询*/
/*老版本的1:1查询*/
SELECT women.NAME AS 妻子, men.NAME AS 丈夫 FROM women , men WHERE  women.husband=men.id;
/*利用连接进行1:1查询*/
/*右连接*/
SELECT women.NAME AS 妻子, men.NAME AS 丈夫 FROM women RIGHT JOIN men ON women.husband=men.id;
/*左连接*/
SELECT women.NAME AS 妻子, men.NAME AS 丈夫 FROM women LEFT JOIN men ON women.husband=men.id;
/*2一对多*/
/*建立表格 初始化*/
CREATE TABLE person2(
id VARCHAR(32) PRIMARY KEY,
NAME VARCHAR(30),
sex CHAR(1),
age INT
);
INSERT INTO person2 VALUES('P1001','小花','0',25);
INSERT INTO person2 VALUES('P1002','张三','1',22);
INSERT INTO person2 VALUES('P1003','Jack','1',24);
INSERT INTO person2 VALUES('P1004','Rose','0',25);
CREATE TABLE car(
id VARCHAR(32) PRIMARY KEY,
NAME VARCHAR(30),
price NUMERIC(10,2),
pid VARCHAR(32),
/*为字段pid定义一个外键约束(来自person2表的字段id)*/
CONSTRAINT car_fk FOREIGN KEY(pid) REFERENCES person2(id)
);
INSERT INTO car VALUES('C001','BMW',80.5,'P1001');
INSERT INTO car VALUES('C002','Benz',100,'P1001');
INSERT INTO car VALUES('C003','BMW',120.05,'P1001');
INSERT INTO car VALUES('C004','Benz',88.5,'P1002');
INSERT INTO car VALUES('C005','QQ',8.5,'P1002');
INSERT INTO car VALUES('C006','BIKE',0.5,'P1003');

/*这时候不能进行删除 因为主键这时是car表格的外键依赖*/
DELETE FROM person2 WHERE(id='P1001');

/*  3、多对多
数据库设计分析
※案例：一个人可以选择多门课程，一门课程又可以被很多人选择。
*/
/*3.1 学生表--实体*/
CREATE  TABLE student(
id VARCHAR(32) PRIMARY KEY,
NAME VARCHAR(30),
age INT
);
/*3.2 课程表--实体*/
CREATE TABLE ject(
id VARCHAR(32) PRIMARY KEY,
NAME VARCHAR(30)
);
/*3.3 关系表--关系*/
CREATE TABLE sj(
studid VARCHAR(32),
subjectid VARCHAR(32)
);
/*3.4 单独添加约束(注意顺序: 要先添加联合主键，再添加外键)*/
/*创建联合主键*/
ALTER TABLE sj ADD CONSTRAINT pk_sj PRIMARY KEY(studid,subjectid);
/*创建两个外键*/
ALTER TABLE sj ADD CONSTRAINT pk_stud FOREIGN  KEY(studid) REFERENCES student(id);
ALTER TABLE sj ADD CONSTRAINT pk_subj FOREIGN KEY(subjectid) REFERENCES ject(id);

/*3.5初始化数据*/
INSERT INTO student VALUES('P001','小花',25);
INSERT INTO student VALUES('P002','Jack',23);
INSERT INTO student VALUES('P003','Tom',24);
INSERT INTO student VALUES('P004','张三',24);
INSERT INTO student VALUES('P005','赵子龙',26);

INSERT INTO ject VALUES('S001','Java');
INSERT INTO ject VALUES('S002','JavaEE');
INSERT INTO ject VALUES('S003','XML');
INSERT INTO ject VALUES('S004','数据库');
INSERT INTO ject VALUES('S005','JQuery');

INSERT INTO sj VALUES('P001','S001');
INSERT INTO sj VALUES('P001','S003');
INSERT INTO sj VALUES('P002','S001');
INSERT INTO sj VALUES('P002','S002');
INSERT INTO sj VALUES('P002','S003');
INSERT INTO sj VALUES('P003','S001');
INSERT INTO sj VALUES('P004','S002');
INSERT INTO sj VALUES('P004','S003');

/*3.6 数据库操作*/
/*关联: 内连接、左关联、右关联*/
/*要求：查询哪些人选了哪些课*/
SELECT student.NAME AS 学生姓名,ject.NAME AS 课程名称 FROM student INNER JOIN sj ON student.id=sj.studid INNER JOIN ject ON sj.subjectid = ject.id;
/*要求：查询哪些人没有选课*/
SELECT student.NAME AS 学生姓名   FROM student LEFT JOIN sj ON student.id=sj.studid LEFT JOIN ject ON sj.subjectid=ject.id WHERE ject.NAME IS NULL;
/*要求：查询哪些课程没有人选*/
SELECT ject.id AS 课程编号, ject.NAME AS 课程名称 FROM student RIGHT JOIN sj ON student.id=sj.studid RIGHT JOIN ject ON sj.subjectid=ject.id WHERE student.name IS NULL;
/*注意：在左右关联中没有绝对，只是看你怎么连表*/