是否有可能通過一個查詢中的不同條件來計數？

Question

我有一個Shipments表，basicly包含日期，裝運數據

ID是integer dateshipped是date

id dateshipped 
1  1-JAN-16 
2  1-JAN-16 
3  3-FEB-16 
4  9-FEB-16 

我想編寫一個查詢其計算基於月的所有貨物。 我應該得到的是：

Jan Feb March.... 
2  2  0 

我知道我可以通過使每個列的查詢做，只得到相關的行此特定月份，只是指望他們。

如下：

Select  (Select count(*) 
      from Shipments 
      Where EXTRACT(YEAR FROM dateshipped)::int=2016 and EXTRACT(MONTH FROM dateshipped)::int=1)as JAN, 
      (Select count(*) 
      from Shipments 
      Where EXTRACT(YEAR FROM dateshipped)::int=2016 and EXTRACT(MONTH FROM dateshipped)::int=2)as FEB 

這工作但其太多相同的代碼...

我wondring是否有可能與一個FROM statment每做列獲得它自己的相關行計數。

喜歡的東西：

Select COL1,COL2,COL3... 
from Shipments 
Where EXTRACT(YEAR FROM dateshipped)::int=2016; 

，並有類似：

COL1 = count only JAN records 
COL2 = count only FEB records 
.... 

也許有一些與Parations上月或任何其他解決方案？

Answer 1

你需要一個支點查詢來實現這一點：

SELECT SUM(CASE WHEN EXTRACT(MONTH FROM dateshipped)::int = 1 THEN 1 ELSE 0 END) AS Jan, 
     SUM(CASE WHEN EXTRACT(MONTH FROM dateshipped)::int = 2 THEN 1 ELSE 0 END) AS Feb, 
     SUM(CASE WHEN EXTRACT(MONTH FROM dateshipped)::int = 3 THEN 1 ELSE 0 END) AS Mar, 
     SUM(CASE WHEN EXTRACT(MONTH FROM dateshipped)::int = 4 THEN 1 ELSE 0 END) AS Apr, 
     SUM(CASE WHEN EXTRACT(MONTH FROM dateshipped)::int = 5 THEN 1 ELSE 0 END) AS May, 
     SUM(CASE WHEN EXTRACT(MONTH FROM dateshipped)::int = 6 THEN 1 ELSE 0 END) AS Jun, 
     SUM(CASE WHEN EXTRACT(MONTH FROM dateshipped)::int = 7 THEN 1 ELSE 0 END) AS Jul, 
     SUM(CASE WHEN EXTRACT(MONTH FROM dateshipped)::int = 8 THEN 1 ELSE 0 END) AS Aug, 
     SUM(CASE WHEN EXTRACT(MONTH FROM dateshipped)::int = 9 THEN 1 ELSE 0 END) AS Sep, 
     SUM(CASE WHEN EXTRACT(MONTH FROM dateshipped)::int = 10 THEN 1 ELSE 0 END) AS Oct, 
     SUM(CASE WHEN EXTRACT(MONTH FROM dateshipped)::int = 11 THEN 1 ELSE 0 END) AS Nov, 
     SUM(CASE WHEN EXTRACT(MONTH FROM dateshipped)::int = 12 THEN 1 ELSE 0 END) AS Dec 
FROM Shipments 
WHERE EXTRACT(YEAR FROM dateshipped)::int=2016 

Answer 2

嘗試的情況下用SUM函數：

Select 
    sum(case when extract(MONTH from dateshipped)=1 then 1 else 0 end) as jan, 
    sum(case when extract(MONTH from dateshipped)=2 then 1 else 0 end) as feb, 
    sum(case when extract(MONTH from dateshipped)=3 then 1 else 0 end) as march 
    ..... 
    ..... 
from Shipments 
Where EXTRACT(YEAR FROM dateshipped)::int=2016; 

Answer 3

由於9.4可以使用FILTER

SELECT 
    count(*) AS total, 
    count(*) FILTER (WHERE Extract(MONTH FROM dateshipped)::int=1) AS JAN, 
    count(*) FILTER (WHERE Extract(MONTH FROM dateshipped)::int=2) AS FEB, 
    ... 
FROM Shipments 
WHERE Extract(YEAR FROM dateshipped)::int=2016;