SQL learning

时间:2023-03-08 16:37:12
SQL learning

一、创建和删除数据库

1、创建用户

//创建用户且置密码,在MySQL中行,但在Oracle中行  ----必须在超级管理员身份下操作

create user hncu identified by '1234'

2、创建数据库

//创建数据库 并手动指定编码格式
//错,因为根据手册的查询,数据库名应该在 EXISTS后面
CREATE DATABASE hncu IF NOT EXISTS DEFAULT CHARACTER SET 'utf8';
//改正:
CREATE DATABASE IF NOT EXISTS hncu DEFAULT CHARACTER SET 'utf8';

3、删除数据库

DROP DATABASE mydb2; //删除数据库
//查询(显示数据库)
SHOW DATABASES; /*注:用Tab键补全代码,类似MyEclipse中的Alt+/ */

二、数据库编码问题

1、指定数据库编码

方法1:可以在MySQL的配置文件如my.ini中指定:

default-character-set = GBK  或

default-character-set = utf8

方法2:在创建数据库时自己指定,如:

CREATE DATABASE IF NOT EXISTS hncu DEFAULT CHARACTER SET 'utf8';

▲相比较而言,对我们来讲,第2种方法更好。一是配置文件是平台,不能随意改,而且通常是由DBA来做的,我们没权设置,甚至MySQL都不在我们的电脑中。二是只

要我们自己在创建数据库时自己指定编码,无论平台默认配置的是什么编码,对我们没有影响,一句话不依赖你的平台----可移置及兼容性好。

2、查看数据库编码

SHOW VARIABLES; //查询系统中的所有变量

SHOW VARIABLES WHERE variable_name LIKE 'character%'; //查询系统中所有的编码方面的参数

查询结果:

character_set_client utf8  //客户端编码 (不同的客户端显示的可能不一样,如cmd窗口和SQLyong)

character_set_connection  utf8  //客户端连接数据库时用的编码

character_set_database gbk  //数据库的默认编码(一般来讲,这是由my.ini配置文件定的。这是没进数据库时的通用编码)

character_set_filesystem binary  //这是数据库自己存储数据文件时用的编码,跟我们关系不大

character_set_results utf8  //查询之后的结果集的编码

character_set_server gbk  //MySQL数据库服务器自己的编码  iso8859-1在这里称Latin1

character_set_system  utf8

character_set_dir   MySQL安装目录 \share\charsets

对我们来讲,为了不出现乱码,必须保证client和connection的编码一致,否则就会乱码。

character_set_database 数据库的编码必须要能够支持中文,否则输入中文有问题的

3、设置(修改)数据编码

//如何设置指定的编码

set character_set_client=gbk;

SET character_set_client=gbk;

SHOW VARIABLES WHERE variable_name LIKE 'character%';

SELECT * FROM stud;//原来的数据显示正常

INSERT INTO stud VALUES(1011,'城院',20,88,'数计学院');

SELECT * FROM stud;//刚刚插入的那条记录,是乱码 //因为我们这里client是gbk,而connection是utf8,不一致了

SET character_set_connection=gbk;//已经把client和connection设成gbk,一致了

INSERT INTO stud VALUES(1012,'城院2',20,88,'数计学院');

SELECT * FROM stud;//显示刚刚插入的那条记录,还是乱码。因为数据库表stud的编码是utf8,而我们客户端与它的连接都是gbk

▲综上,client、connection、我们所访问的数据库的编码  这三者都要一致且应该是支持中文的编码

上面只能保证添加到数据库中的汉字不会出现乱码。

▲如果读取出来,还要看results、所访问的数据库的编码和我们java代码中的解码是否一致。

三、创建表格

//创建表格

CREATE TABLE stud(

id INT PRIMARY KEY,

sname VARCHAR(30) ,

age INT);

-----------------------------------------------------------------------
varchar(20)  ----可变的char数组,类似Java当中的String

char(20)   ----固定长度的char数组

//SQL语言 全部忽略大小写----大小写不敏感

-----------------------------------------------------------------------

四、对表格操作

USE hncu;

SHOW TABLES;//查询(显示)表格

DESC stud;  //查看数据表student的表结构

1、往数据表中插入数据

INSERT INTO stud VALUES( 1003,'Rose',22 );

//插入记录,当数据不全时,要指定列名

INSERT INTO stud(id,sname,age,score) VALUES(1010,'李小明',25,90);//未指定列名的方式赋值时,必须要给全,默认值不给也不行

INSERT INTO a(id,sname) VALUES(1,'Tom'); //错:UNIQUE限定该列的值必须唯一(可以为NULL,但最多只能有一个)

INSERT INTO a(id,sname) VALUES(2,'Tom'); //对,性别未赋,则用默认
//性能优化:指定列名的方式性能更好!

2、查询数据

SELECT * FROM stud;//查询表中所有的数据

SELECT sname,age FROM stud; //只查询(显示)表中的指定列

3、删除数据

DELETE FROM stud WHERE age=30; //删除年龄为30的表记录

4、更新数据

UPDATE stud SET sname='杰克' WHERE sname='Jack'; //更改数据

5、更改表结构ALTER TABLE (添加一列)

ALTER TABLE stud ADD COLUMN score NUMERIC(4,2);

※※※※※ 建立联合主键 ※※※※※
ALTER TABLE sj ADD CONSTRAINT sj_pk PRIMARY KEY(studId,jectId);
//添加外键约束1(为sj表的studId字段添加外键student(id)约束)
ALTER TABLE sj ADD CONSTRAINT sj_fk1 FOREIGN KEY(studId) REFERENCES student(id);
//添加外键约束2(为sj表的jectId字段添加外键ject(id)约束)
ALTER TABLE sj ADD CONSTRAINT sj_fk2 FOREIGN KEY(jectId) REFERENCES ject(id);

6、创建视图

CREATE VIEW studView AS SELECT * FROM stud WHERE score>=60;

SELECT * FROM studview;

五、对表查询操作中的那些事

INSERT INTO stud VALUES(1004,'张三',38,60);

INSERT INTO stud VALUES(1005,'王三',30,60);

INSERT INTO stud VALUES(1006,'王五',30,60);

INSERT INTO stud VALUES(1007,'王五六',30,80);

※1、范围查询

//查询年龄在24-26之间的学生信息

1)连续区间内的查询

SELECT * FROM stud WHERE age>=24 AND age<27;

2)用BETWEEN是左右都包含,如下功能是[24,26]

SELECT * FROM stud WHERE age BETWEEN 24 AND 26;

3)离散的多个

SELECT * FROM stud WHERE age=24 OR age=38;

SELECT * FROM stud WHERE age IN(24,26,30);

※2、模糊查询LIKE  参数:%(任意匹配), _(匹配一个)

//查询姓“王”的学生信息

SELECT * FROM stud WHERE sname LIKE '王%'

//查询姓“王”且名为单字的学生信息

SELECT * FROM stud WHERE sname LIKE '王_';

//查询姓“王”且名为双字的学生信息

SELECT * FROM stud WHERE sname LIKE '王__';

//查询姓名中包含“五”字的学生信息

SELECT * FROM stud WHERE sname LIKE '%五%';

※3、范围查询和模糊查询联合

//查询姓名中包含“五”字 且 年龄大于30 的学生信息

SELECT * FROM stud WHERE sname LIKE '%五%' AND age>30;

※4、空值查询

//查询无名英雄学生的信息(VARCHAR)

SELECT * FROM stud WHERE sname IS NULL;

//查询没有年龄信息的学生

//错:SELECT * FROM stud WHERE age==NULL;

SELECT * FROM stud WHERE age IS NULL;

※5、聚合函数

1)COUNT 统计表格的行数

SELECT COUNT(*) AS TEMPTABLE FROM stud;//as temptable 含义:就是将查询出的结果(表格的行数)另命名为:temptable

SELECT COUNT(1) AS TEMPTABLE FROM stud;

//统计有年龄值的学生人数

SELECT COUNT(age) AS TEMPTABLE FROM stud;

//统计有年龄值且有分数值的学生人数

SELECT COUNT(age) AS TEMPTABLE FROM stud WHERE score IS NOT NULL;

2)AVG 统计平均分且取整(注:AVG函数只统计非NULL的数据记录)

SELECT ROUND(AVG(score)) FROM stud; //这种方式一般不用,因为列名是自动生成的,我们在程序中不好访问

SELECT ROUND(AVG(score)) AS averageScore FROM stud;

3)SUM 分数求和

SELECT SUM(score) AS ss FROM stud;

4)MAX 年龄最大值

SELECT MAX(age) AS maxAge FROM stud;

※6 、WHERE子句+ IN子句

//查询年龄最小的那个人的名字

SELECT sname FROM stud WHERE age=(SELECT MIN(age) FROM stud);

SELECT sname FROM stud WHERE age IN(SELECT MIN(age) FROM stud);

※7、排序

SELECT * FROM stud GROUP BY age ASC; //不重复排序(即年龄相同的,只显示第一个0

SELECT * FROM stud GROUP BY age ASC; //显示出所有年龄段

SELECT * FROM stud ORDER BY age ASC; //普通排序--升序

SELECT * FROM stud ORDER BY age DESC; //普通排序--降序

※8、distinct(不重复的值)

SELECT DISTINCT sname,age FROM stud GROUP BY age DESC;

※9、EXISTS() 判断括号内的内容是否存在----注意,下面的例子,只要存在年龄为26的学生,就会输出所有数据

SELECT * FROM stud WHERE EXISTS( SELECT * FROM stud WHERE age=26 );

※10、演示分组

ALTER TABLE stud ADD COLUMN dept VARCHAR(20);

UPDATE stud SET dept='信息学院' WHERE score>=65;

UPDATE stud SET dept='通信学院' WHERE score=60;

UPDATE stud SET dept='土木学院' WHERE score<60;

SELECT * FROM stud;

//分组计算

//按学院计算平均分(每个学员的平均分)

SELECT dept,AVG(score) AS '学院平均分' FROM stud GROUP BY dept;

※11、字符串处理函数

SELECT * FROM stud WHERE sname='AAA';

SELECT * FROM stud WHERE TRIM(sname)='AAA';//去掉左右的空格

SELECT * FROM stud WHERE LTRIM(RTRIM(sname))='AAA';//和上面等价

UPDATE stud SET dept='数计学院' WHERE id=1011;

SELECT LEFT(TRIM(sname),2) FROM stud; //取去掉空格后的左起2个字符

SELECT REVERSE(TRIM(sname)) FROM stud;

CREATE TABLE person(
  id INT,
  sname VARCHAR(30) ,
  age INT
);
ALTER TABLE person ADD CONSTRAINT person_pk PRIMARY KEY(id);//更灵活

DROP TABLE person;

CREATE TABLE person(

id INT PRIMARY KEY,

sname VARCHAR(30) NOT NULL,

age INT

); //用NOT NULL 限制非空输入

※12、性别字段(例如:数据库存0、1 而显示出来为男、女)

一般不定义成BOOLEAN型,因为有的数据库不支持,为考虑兼容,通常用CHAR(1)

CREATE TABLE a(

id INT UNIQUE,

sname VARCHAR(10),

sex CHAR(1) DEFAULT '0'

);

//显示性别 (真实值与显示值之间的转换)

SELECT * FROM a;

SELECT id,sname,(CASE sex WHEN '0' THEN '女' WHEN '1' THEN '男' ELSE 'Null' END) xb FROM a;

SELECT id,sname,(CASE sex WHEN '0' THEN '女' WHEN '1' THEN '男' ELSE 'Null' END) AS xb FROM a;

SELECT id,sname,(CASE WHEN sex='0' THEN '女' WHEN sex='1' THEN '男' ELSE 'Null' END) AS xb FROM a;

※13、无关子查询

//需求:具有同龄人的学生

SELECT * FROM stud;

SELECT * FROM stud WHERE age IN ( SELECT age FROM stud GROUP BY age HAVING COUNT(age)>=2 ) ORDER BY age DESC;

//练练别名

SELECT * FROM stud AS xs WHERE age IN ( SELECT age FROM xs GROUP BY age HAVING COUNT(age)>=2 ) ORDER BY age DESC;

//需求2:不但具有同龄人,而且年龄大于等于30的学生

//法1

SELECT * FROM stud WHERE age IN ( SELECT age FROM stud GROUP BY age HAVING COUNT(age)>=2 AND age>=30 ) ORDER BY age DESC;

//法2

SELECT * FROM stud WHERE age>=30 AND age IN ( SELECT age FROM stud GROUP BY age HAVING COUNT(age)>=2 ) ORDER BY age DESC;

※14、固定搭配

SELECT * FROM  +  WHERE  + ORDER BY(要放在最后)

GROUP BY + HAVING

※15、关系查询

-----------------------------------------
DROP TABLE person;

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)一对一关系的操作:查出每对夫妻的姓名

CREATE VIEW w AS SELECT * FROM person WHERE sex='0';

CREATE VIEW m AS SELECT * FROM person WHERE sex='1';

//不利用表与表之间的关系

SELECT w.NAME AS 妻子, m.NAME AS 丈夫 FROM w,m WHERE w.husband=m.id AND m.wife=w.id;

//现在更先进的方式:利用表间的关系

SELECT w.NAME AS 妻子, m.NAME AS 丈夫 FROM w INNER JOIN m ON w.husband=m.id AND m.wife=w.id;

SELECT * FROM person;

2)一对多的关系  代码演示

//步骤1:画E-R图

//步骤2:分别建实体表,并给多方的表添加外键约束

CREATE TABLE person2(

id VARCHAR(32) PRIMARY KEY,

pname VARCHAR(30),

sex CHAR(1)

);

CREATE TABLE car(

id VARCHAR(32) PRIMARY KEY,

cname VARCHAR(30),

price NUMERIC(10,2),

pid VARCHAR(32),

CONSTRAINT car_fk FOREIGN KEY(pid) REFERENCES person2(id)

);

DROP TABLE car;

//步骤3:为两个表添加测试数据

//实体表1

INSERT INTO person2(id,pname,sex) VALUES('P001','Jack','1');

INSERT INTO person2(id,pname,sex) VALUES('P002','Tom','1');

INSERT INTO person2(id,pname,sex) VALUES('P003','Rose','0');

INSERT INTO person2(id,pname,sex) VALUES('P004','Mary','0');

INSERT INTO person2(id,pname,sex) VALUES('P005','Mike','1');

SELECT * FROM person2;

////实体表2

INSERT INTO car(id,cname,price,pid) VALUES('C001','BMW',123.5,'P001');

INSERT INTO car(id,cname,price,pid) VALUES('C002','Benz',123.5,'P001');

INSERT INTO car(id,cname,price,pid) VALUES('C003','BMW',223.5,'P001');

INSERT INTO car(id,cname,price,pid) VALUES('C011','BMW',83.5,'P003');

INSERT INTO car(id,cname,price,pid) VALUES('C012','Benz',100,'P003');

INSERT INTO car(id,cname,price,pid) VALUES('C013','Audi',223.5,'P003');

INSERT INTO car(id,cname,price,pid) VALUES('C021','BMW',88.5,'P004');

INSERT INTO car(id,cname,price,pid) VALUES('C022','QQ',10,'P004');

INSERT INTO car(id,cname,price,pid) VALUES('C023','Audi',73,'P005');

INSERT INTO car(id,cname,price) VALUES('C033','Audi',1000);

//该句代码执行错误,因为编号为P006的人在Person2表中不存在,这就是参照完整性

INSERT INTO car(id,cname,price,pid) VALUES('C033','Audi',1000,'P006');
SELECT * FROM car;

//查询:哪些人有什么样的车 (用"表名.列名"的形式访问列,如果列名不重复,可以省略表名)

//利用一方的主键和“多方”的外键进行关联

SELECT person2.pname,car.cname FROM person2,car WHERE person2.id=car.pid;

//查询Jack有什么车

SELECT person2.pname,car.cname FROM person2,car WHERE person2.id=car.pid AND person2.pname='Jack' ;

//查询哪些人有两辆以上的车

SELECT person2.pname,COUNT(pname) AS 车数量 FROM person2,car WHERE person2.id=car.pid GROUP BY pname HAVING COUNT(pname)>=2 ORDER BY 车

数量;

SELECT * FROM person2 WHERE id IN ( SELECT pid FROM car GROUP BY pid HAVING COUNT(pid)>=2 );

※16、关联查询

//查询哪些人没有车

SELECT * FROM person2 WHERE id NOT IN( SELECT pid FROM car );

//用左关联(LEFT JOIN)来查询:哪些人有什么样的车(没车的也是一种情况,要显示)

SELECT person2.pname,car.cname,car.price FROM person2 LEFT JOIN car ON person2.id=car.pid ORDER BY person2.id;

//用内关联(INNER JOIN)来查询:哪些人有什么样的车(没车的不显示)

SELECT person2.pname,car.cname,car.price FROM person2 INNER JOIN car ON person2.id=car.pid ORDER BY person2.id;

//查询每辆车的销售情况(如果有主人就显示,没有则显示NULL)

SELECT person2.pname,car.cname,car.price FROM person2 RIGHT JOIN car ON person2.id=car.pid ORDER BY person2.id;

(+在左边时  右关联 ,+右边时 左关联)

--------------------------------------------------
DELETE FROM person2 WHERE id='P005';

CREATE TABLE student(

id VARCHAR(32) PRIMARY KEY,

NAME VARCHAR(30),

age INT

);

CREATE TABLE ject(

id VARCHAR(32) PRIMARY KEY,

NAME VARCHAR(30),

price NUMERIC(5,2)

);

CREATE TABLE sj(

studId VARCHAR(32) NOT NULL,

jectId VARCHAR(32)

);

//建立联合主键

ALTER TABLE sj ADD CONSTRAINT sj_pk PRIMARY KEY(studId,jectId);

//添加外键约束1(为sj表的studId字段添加外键student(id)约束)

ALTER TABLE sj ADD CONSTRAINT sj_fk1 FOREIGN KEY(studId) REFERENCES student(id);

//添加外键约束2(为sj表的jectId字段添加外键ject(id)约束)

ALTER TABLE sj ADD CONSTRAINT sj_fk2 FOREIGN KEY(jectId) REFERENCES ject(id);

//添加测试数据

//学生表

INSERT INTO student(id,NAME,age) VALUES('S001','Jack',25);

INSERT INTO student(id,NAME,age) VALUES('S002','Tom',24);

INSERT INTO student(id,NAME,age) VALUES('S003','张三',23);

INSERT INTO student(id,NAME,age) VALUES('S004','李四',24);

INSERT INTO student(id,NAME,age) VALUES('S005','Rose',25);

SELECT * FROM student;

//课程表

INSERT INTO ject(id,NAME,price) VALUES('J001','Java',25);

INSERT INTO ject(id,NAME,price) VALUES('J002','MySQL',30);

INSERT INTO ject(id,NAME,price) VALUES('J003','Oracle',55.9);

INSERT INTO ject(id,NAME,price) VALUES('J004','软件工程',20.25);

INSERT INTO ject(id,NAME,price) VALUES('J005','WEB开发',125);

SELECT * FROM ject;

//选课表

INSERT INTO sj(studId,jectId) VALUES('S001','J001');

INSERT INTO sj(studId,jectId) VALUES('S001','J002');

INSERT INTO sj(studId,jectId) VALUES('S001','J003');

INSERT INTO sj(studId,jectId) VALUES('S002','J001');

INSERT INTO sj(studId,jectId) VALUES('S002','J003');

INSERT INTO sj(studId,jectId) VALUES('S003','J001');

INSERT INTO sj(studId,jectId) VALUES('S003','J002');

INSERT INTO sj(studId,jectId) VALUES('S004','J003');

INSERT INTO sj(studId,jectId) VALUES('S005','J001');

SELECT * FROM sj;

应用测试

//查询哪些人选了哪些课 ----要求显示:人名,课程名

//采用的是92标准

SELECT student.name,ject.NAME FROM student,ject,sj WHERE student.id=sj.studId AND sj.jectId=ject.id;

//采用96标准

SELECT student.name,ject.NAME FROM student

INNER JOIN sj ON student.id=sj.studId

INNER JOIN ject ON sj.jectId=ject.id;

//查询哪些人没有选课(左关联)

SELECT student.name,ject.NAME FROM student

LEFT JOIN sj ON student.id=sj.studId

LEFT JOIN ject ON sj.jectId=ject.id WHERE ject.NAME IS NULL;

//查询哪些课没人选(右关联)

SELECT student.name,ject.NAME FROM student

RIGHT JOIN sj ON student.id=sj.studId

RIGHT JOIN ject ON sj.jectId=ject.id WHERE student.NAME IS NULL;

//左右关联可以相互转换,如把上面的代码用左关联

SELECT student.name,ject.NAME FROM jec

LEFT JOIN sj ON ject.id=sj.jectId

LEFT JOIN student ON sj.studId=student.id WHERE student.NAME IS NULL

六、存储过程

//定义存储过程p1 // “DELIMITER

”这句是为了让解析器把“

”当作结束标志(否则默认是把“;“号当作结束标记),这样存储过程中的语句结束符";"就不会当作过程的结束

记。

DELIMITER$$

CREATE PROCEDURE p1()

BEGIN

SELECT * FROM stud;

INSERT INTO stud(id,sname,age,score,dept) VALUES(1014,'刘三丰',33,55,'通信学院');

END$$

DELIMITER;   //把结束标记还原回来

CALL p1();  //调用存储过程p1

1、带参数的存储过程

DELIMITER$$

CREATE PROCEDURE p2(IN id INT, IN nm VARCHAR(30) )

BEGIN

INSERT INTO stud(id,sname) VALUES(id,nm);

END$$

DELIMITER ;

DROP PROCEDURE p2;

CALL p2(1015,'吊丝');

2、有返回值的存储过程----参数与变量问题( @变量名  ,一个@为用户变量,两个@即 @@为全局的系统变量 )

DELIMITER$$

CREATE PROCEDURE p3(IN id INT, IN nm VARCHAR(30), OUT num INT )

BEGIN

INSERT INTO stud(id,sname) VALUES(id,nm);

SELECT COUNT(*) INTO num FROM stud;

END$$

DELIMITER ;

CALL p3(1016,'无名',@aa);

SELECT @aa; //输出变量aa的值

七、事务处理

START TRANSACTION

DELETE FROM stud WHERE id=1015;

DELETE FROM stud WHERE id=1014;

SELECT * FROM stud;

ROLLBACK /  COMMIT;

◎采用事务的java编程

try{

st.execute("START TRANSACTION;");

st.execute("DELETE FROM stud WHERE id=1015;");

st.execute("DELETE FROM stud WHERE id=1014;");

....

st.execute(" commit();  ");

}catch(Exception e){

rollback();
}