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
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
//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
//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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