如何为每列使用不同的WHERE子句

Question

我认为这应该很简单，但我不知道该怎么做。

我想编写一个输出三列的SQL查询：周，苹果数和橙子数。

说我的数据库看起来像这样：

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

我希望查询输出：

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

我可以像这样单独购买苹果和橘子：

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)

但显然更愿意使用相同的查询生成两个列。

Answer 1

我不确定date_trunc是什么，所以我把它排除在外。您可以使用JOIN命令来连接查询，也可以通过将SUM与CASE结合使用单个查询来执行：

SELECT week,
SUM(CASE WHEN fruit_type = 'Apples' THEN fruit_num END) as 'Apples',
SUM(CASE WHEN fruit_type = 'Oranges' THEN fruit_num END) as 'Oranges'
from fruit
group by week
order by week

示例：

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

Answer 2

您可以使用案例陈述。您在执行select时应用条件，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)