hive的dual表

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dual表的概念来自oracle，数据库建立时即与数据字典一起初始化，该表只有一个varchar2类型名为dummy的字段，表数据只有一行“X”，用来查询一些系统信息，
如

select sysdate from dual; select user from dual;select seq.nextval from dual

等。
为了能在hive中测试一些时间、数学、聚合函数，可以仿照oracle创建dual表。

hive>create table dual (dummy String);

创建一个仅包含X的文件

echo X > /home/hadoop/dual.txt

hive>load data local inpath '/home/hadoop/dual.txt' overwrite into table dual;

即可使用。如：

hive> select round(3.1415,2) from dual;OK3.14Time taken: 0.617 seconds, Fetched: 1 row(s)

hive> select rand() from dual;OK0.539591509499207Time taken: 0.138 seconds, Fetched: 1 row(s)hive> select rand(1) from dual;OK0.7308781907032909Time taken: 0.153 seconds, Fetched: 1 row(s)hive> select rand(1) from dual;OK0.7308781907032909Time taken: 0.183 seconds, Fetched: 1 row(s)hive> select rand(1) from dual;OK0.7308781907032909Time taken: 2.161 seconds, Fetched: 1 row(s)hive> select rand() from dual;OK0.663918139786933Time taken: 0.143 seconds, Fetched: 1 row(s)

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