动态列名称在视图（Postgres）

Question

我目前正在编程一个SQL视图应该提供一个特定月份的填充字段的计数。

这是我想建造的观点：

Country | (Current Month - 12) Eg Feb 2011 | (Current Month - 11) | (Current Month - 10) 
----------|----------------------------------|----------------------|--------------------- 
UK  | 10        | 11     | 23 

月份下的数字应该是某个国家的所有填充字段的计数。该字段被命名为eldate，并且是格式为10-12-2011的日期（格式为char）。我希望计数只计算与月份相匹配的日期。

因此，“当前月份 - 12”栏应该只包括现在12个月内的月份数。例如本月 - 英国的12应包括2011年2月之内的日期计数。

我想列标题实际上反映了一个月它正在寻找这样：

Country | Feb 2011 | March 2011 | April 2011 
--------|----------|------------|------------ 
UK  | 4  | 12   | 0 

因此，像：

SELECT c.country_name, 
     (SELECT COUNT("C1".eldate) FROM "C1" WHERE "C1".eldate LIKE %NOW()-12 Months% AS NOW() - 12 Months 
     (SELECT COUNT("C1".eldate) FROM "C1" WHERE "C1".eldate LIKE %NOW()-11 Months% AS NOW() - 11 Months 
FROM country AS c 
INNER JOIN "site" AS s using (country_id) 
INNER JOIN "subject_C1" AS "C1" ON "s"."site_id" = "C1"."site_id" 

显然，这并不工作，但只给你一个关于我所得到的想法。

任何想法？

感谢您的帮助，请再问问。

Answer 1

你可以截断日期使可比性：

WHERE date_trunc('month', eldate) = date_trunc('month', now()) - interval '12 months' 

UPDATE

这种替代你的查询：

(SELECT COUNT("C1".eldate) FROM "C1" WHERE date_trunc('month', "C1".eldate) = 
    date_trunc('month', now()) - interval '12 months') AS TWELVE_MONTHS_AGO 

但是，这将涉及到表扫描对于每个月，所以你可以做一个更多的沿着这些线扫描：

SELECT SUM(CASE WHEN date_trunc('month', "C1".eldate) = date_trunc('month', now()) - interval '12 months' THEN 1 ELSE 0 END) AS TWELVE_MONTHS_AGO 
     ,SUM(CASE WHEN date_trunc('month', "C1".eldate) = date_trunc('month', now()) - interval '11 months' THEN 1 ELSE 0 END) AS ELEVEN_MONTHS_AGO 
... 

或者与其他人一起显示月份的表格。

UPDATE2

继从一月列固定至12月的评论，我想是这样的：过滤器上值得记录的最后几年，然后在相应月份总结。也许是这样的：

SELECT SUM(CASE WHEN EXTRACT(MONTH FROM "C1".eldate) = 1 THEN 1 ELSE 0 END) AS JAN 
     ,SUM(CASE WHEN EXTRACT(MONTH FROM "C1".eldate) = 2 THEN 1 ELSE 0 END) AS FEB 
     ... 

    WHERE date_trunc('month', "C1".eldate) < date_trunc('month', now()) 
    AND date_trunc('month', "C1".eldate) >= date_trunc('month', now()) - interval '12 months' 

Answer 2

我的第一个倾向是产生该表：

+---------+-------+--------+ 
| Country | Month | Amount | 
+---------+-------+--------+ 
| UK  | Jan | 4  | 
+---------+-------+--------+ 
| UK  | Feb | 12  | 
+---------+-------+--------+ 

等，并转动它。所以，你会用（例如）开始：

SELECT 
    c.country, 
    EXTRACT(MONTH FROM s.eldate) AS month, 
    COUNT(*) AS amount 
FROM country AS c 
JOIN site AS s ON s.country_id = c.id 
WHERE 
    s.eldate > NOW() - INTERVAL '1 year' 
GROUP BY c.country, EXTRACT(MONTH FROM s.eldate); 

然后，您可以把它插入到one the crosstab functions from the tablefunc module达到支点，做这样的事情：

SELECT * 
FROM crosstab('<query from above goes here>') 
    AS ct(country varchar, january integer, february integer, ... december integer);