Postgres：每分鐘數據的總計平均數

Question

我目前有一個查詢，它返回表格中的日期時間和值。

Select time, value from table; 

2014-02-05 15:49:54.458 70 
2014-02-05 15:49:55.46 70 
2014-02-05 15:49:56.483 70 
2014-02-05 15:49:57.487 70 
2014-02-05 15:49:58.503 70 
2014-02-05 15:50:00.042 70 
2014-02-05 15:50:01.546 70 
2014-02-05 15:50:03.056 70 
2014-02-05 15:50:04.093 70 

我所希望做的是一樣的東西

select average(value) from table group by (cast time to min resolution)time; 

2014-02-05 15:49 70 
2014-02-05 15:50 70 

Answer 1

這應做到：

select date_trunc('minute', time), 
     avg(value) 
from the_table 
group by date_trunc('minute', time) 
order by date_trunc('minute', time); 

更多細節的手冊中：http://www.postgresql.org/docs/current/static/functions-datetime.html#FUNCTIONS-DATETIME-TRUNC

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^{Btw：time和value是一列可怕的名字}