hive的dual表

dual表的概念來自oracle，數據庫建立時即與數據字典一起初始化，該表只有一個varchar2類型名爲dummy的字段，表數據只有一行“X”，用來查詢一些系統信息，
如

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

等。
爲了能在hive中測試一些時間、數學、聚合函數，可以仿照oracle創建dual表。

hive>create table dual (dummy String);

創建一個僅包含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;
OK
3.14
Time taken: 0.617 seconds, Fetched: 1 row(s)

hive> select rand() from dual;
OK
0.539591509499207
Time taken: 0.138 seconds, Fetched: 1 row(s)
hive> select rand(1) from dual;
OK
0.7308781907032909
Time taken: 0.153 seconds, Fetched: 1 row(s)
hive> select rand(1) from dual;
OK
0.7308781907032909
Time taken: 0.183 seconds, Fetched: 1 row(s)
hive> select rand(1) from dual;
OK
0.7308781907032909
Time taken: 2.161 seconds, Fetched: 1 row(s)
hive> select rand() from dual;
OK
0.663918139786933
Time taken: 0.143 seconds, Fetched: 1 row(s)