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HomeDatabaseMysql TutorialOracle中提取主键拼字符串的问题

最近在做一个数据库是用的Oracle的项目,也是我的第一个项目模块,感觉很蛋疼,但是有很多收获。下面就是我遇到的问题和解决方法

最近在做一个数据库是用的Oracle的项目,也是我的第一个项目模块,感觉很蛋疼,但是有很多收获。下面就是我遇到的问题和解决方法,我是菜鸟级,希望高手如果发现错误,请指出。

比如:

创建的Oracle的表,主键描述:“四位树形代码”

比如用户表中:

表名:USER

表记录:

USER_ID CHAR(15) NOT NULL,

USER_NAME VARCHAR2(20) NOT NULL,

USER_EMAIL VARCHAR2(30),

PRIMARY KEY(USER_ID)

 

先插入一条数据:

INSERT INTO USER VALUES('0001', '张三', '123456789@');

 

现在我要查询:

SELECT * FROM USER;

 

执行的结果:

USER_ID                             USER_NAME                                  USER_EMAIL

   0001                                          张三                                       123456789@

 

如果我们从Oracle中只查询出USER_ID

SELECT USER_ID

FROM USER;

 

执行的结果:

USER_ID

   0001

 

但是在我们拼字符串sql的时候,比如我们要联表查询,USER_ID在另外一个表中是一个内联的字段,我们就要在USER_ID内联的表中使用到。

比如在“学生表”中:

表名:STUDENT

表记录:

STUDENT_ID CHAR(15) NOT NULL,

USER_ID CHAR(15) NOT NULL,

STUDENT_NAME VARCHAR2(20) NOT NULL,

PRIMARY KEY(STUDENT_ID)

 

插入一条记录:

INSERT INTO STUDENT VALUES('0001','0001' , '学生张三');

查询(这里的查询,我要信息中给学生分配邮箱信息)

SELECT S.STUDENT_NAME, U.USER_MAIL

FROM STUDENT S, USER U

WHERE S.USER_ID=U.USER_ID;

 

执行结果:

STUDENT_NAME                                               USERMAIL

     学生张三                                                 123456789@163..com

重点:

在数据库中,直接这样用肯定是对的,如果我们在java文件中使用字符串sql进行拼字符串就会出现问题了。

使用JDBC连接,执行SQL,返回查询结果,这里省略JDBC的处理

假设使用public List baseListSql(String sql); 方法处理。

拼sql的方法:

(1)

//这一句中user_name是传入的参数,根据传入的user的名字找到user_id的值

String usr_id = this.baseListSql("SELECT USER_ID FROM USER WHERE USER_NAME='"+user_name+"' ");  

 //根据user_id关联USER表中的USER_MAIL

String sql = "SELECT S.STUDENT_NAME, U.USER_MAIL FROM STUDENT S, USER U WHERE S.USER_ID='"+user_id+"'";

//通过JDBC执行后返回List

List students = this.baseListSql(sql);

//返回数据

return  students;

分析:

直接看的时候,还认为这样的sql逻辑是对的,字符串也拼对了的,但是是不能执行成功的,当然你可以使用System.out.println(sql);和Debug的模式进行调试,但是我这里记下,不想每次都因为这个错误去调试半天。

这里USER_ID是CHAR类型,长度是15 , 我们使用“四位树形代码”输入的数据是'0001'这样当他查出来是'0001           ',后面全部都是空格,所以在下面拼的sql中传入的值就不正确了,,所以需要一定的处理,处理如方案(2)

(2)

//这一句中user_name是传入的参数,根据传入的user的名字找到user_id的值

String usr_id = this.baseListSql("SELECT USER_ID FROM USER WHERE USER_NAME='"+user_name+"' ");  

//首先使用String的trim()方法去除所有空格,这样就正确了

user_id = user_id.trim();

 //根据user_id关联USER表中的USER_MAIL

String sql = "SELECT S.STUDENT_NAME, U.USER_MAIL FROM STUDENT S, USER U WHERE S.USER_ID='"+user_id+"'";

//通过JDBC执行后返回List

List students = this.baseListSql(sql);

//返回数据

return  students;

举例中可能会有错误,希望发现错误指出一下。
这样就可以执行成功了!找的时候没注意,幸苦死了,记录一下pass一下它!

下次有时间继续记录遇到的问题,哈哈!奖励一下自己!遇到得多解决得多,到时候一定要成为高手。

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