如何爲每列使用不同的WHERE子句

Question

我認爲這應該很簡單，但我不知道如何去做。

我想編寫一個SQL查詢，它將輸出三列：星期，蘋果數量和橙子數量。

說我的數據庫看起來是這樣的：

week    | fruit_type | fruit_num 
------------------------------------------ 
6/17/2013 0:00:00 | Apples  | 4 
6/17/2013 0:00:00 | Oranges | 3 
6/24/2013 0:00:00 | Oranges | 1 
6/24/2013 0:00:00 | Apples  | 2 
6/24/2013 0:00:00 | Apples  | 7 

我想查詢輸出這樣的：

week    | Apples | Oranges 
------------------------------------------ 
6/17/2013 0:00:00 | 4  | 3 
6/24/2013 0:00:00 | 9  | 1 

我能得到蘋果和橘子分別是這樣的：

select 
date_trunc('week',week) as "Week", 
sum(fruit_type) as "Apples" 
from table 
where fruit_type = 'Apples' 
group by date_trunc('week',week) 
order by date_trunc('week',week) 

但顯然更喜歡用相同的查詢生成兩列。

Answer 1

我不知道的，所以我留下什麼date_trunc了這一點。你應該使用 -

http://sqlfiddle.com/#!2/69bfb/2

Answer 2

你可以使用case語句。您在執行選擇時應用條件，因此不需要where子句。

SELECT 
date_trunc('week',week) as "Week", 
[Apples] = SUM(CASE WHEN fruit_type = 'Apples' THEN fruit_num END), 
[Oranges] = SUM(CASE WHEN fruit_type = 'Oranges' THEN fruit_num END) 

FROM 
table 

GROUP BY ... 

Answer 3

select 
date_trunc('week',week) as "Week", 
CASE WHEN fruit_type = 'Apples' THEN sum(fruit_type) as "Apples", 
CASE WHEN fruit_type = 'oranges' THEN sum(fruit_type) as "Oranges" 
from table 
group by date_trunc('week',week) 
order by date_trunc('week',week)