Postgres generate_series

Question

我想要的是对一个表进行统计，为此我使用了generate_series();

下面是我正在做的事情：

SELECT x.month, amount
FROM (SELECT generate_series(
                 min(date_trunc('month', date)),
                 max(date_trunc('month', date)),
                 '1 month'
      ) AS month
      FROM table
      WHERE user_id = 55 AND ...
) x
LEFT JOIN (
      SELECT SUM(amount) AS amount, date_trunc('month', date) AS month
      FROM table
      WHERE user_id = 55 AND ...
      GROUP BY month
) q ON q.month = x.month
ORDER BY month

这很有效，但当我想要应用过滤器时，比如获取特定用户的数量，我必须应用它们两次。有没有一种方法可以避免过滤两次，或者用一种更有效的方式重写，因为我不确定这是否是正确的方法？

Answer 1

您可以为此编写WITH query：

WITH month_amount AS
(
    SELECT
        sum(amount) AS amount,
        date_trunc('month', date) AS month
    FROM Amount
    WHERE user_id = 55 -- AND ...
    GROUP BY month
)
SELECT month, amount
FROM
    (SELECT generate_series(min(month), max(month), '1 month') AS month
    FROM month_amount) x
LEFT JOIN month_amount
USING (month)
ORDER BY month;

示例结果：

SELECT * FROM amount WHERE user_id = 55;
 amount_id | user_id | amount |    date    
-----------+---------+--------+------------
         3 |      55 |      7 | 2011-03-16
         4 |      55 |      5 | 2011-03-22
         5 |      55 |      2 | 2011-05-07
         6 |      55 |     18 | 2011-05-27
         7 |      55 |      4 | 2011-06-14
(5 rows)

WITH month_amount ..
         month          | amount 
------------------------+--------
 2011-03-01 00:00:00+01 |     12
 2011-04-01 00:00:00+02 |       
 2011-05-01 00:00:00+02 |     20
 2011-06-01 00:00:00+02 |      4
(4 rows)

Answer 2

您可以在WITH子句中执行查询，然后使用SELECT添加缺少的月份：

WITH query AS (
      SELECT SUM(amount) AS amount, date_trunc('month', date) AS month
      FROM table
      WHERE user_id = 55 AND ...
      GROUP BY month
)
SELECT date(d.month) AS month, q.amount
FROM (
    SELECT generate_series(min(month), max(month), '1 month') AS month
    FROM query
    ) d
    LEFT JOIN query q ON q.month = d.month
ORDER BY month