33 lines
917 B
Markdown
33 lines
917 B
Markdown
# dual表的使用
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### 简介
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dual 是一张虚拟表用来构成select语句的语法规则，oracle保证dual里面只有一条记录。
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## dual用途
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* 查看当前用户
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```sql
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select user from dual;
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```
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* 用来调用系统函数
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```sql
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select to_char(sysdate,'yyyy-mm-dd hh24:mi:ss') from dual; -- 获取当前时间
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select SYS_CONTEXT('USERENV','TERMINAL') FROM DUAL; -- 获取当前登陆的用户
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select SYS_CONTEXT('USERENV','language') from dual; -- 获取当前的loacle
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select dbms_random.random from dual; -- 获取随机数
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```
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* 得到当前序列的值或者下一个值
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```sql
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select your_sequence.nextval from dual;--获得序列your_sequence的下一个值
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select your_sequence.currval from dual;--获得序列your_sequence的当前值
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```
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* 得到1到(n - 1)的数
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```sql
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select level r from dual connect by level < 10; -- 输出1-9的数
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```
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* 计算器
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```sql
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select 7*9 from dual;
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```
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