Select With 语句中为 Null

Question

我在 SQLWorkbenchJ 中使用 PostgreSQL，但遇到困难。

我有一个 WITH 声明 select 日期取决于它的行号。如果语句找不到行号，我想在日期字段中 select null。这目前不会发生，它只是 selects 记录，其中所有字段都不为空。我假设它与连接有关，但我不确定。

现声明如下。它应该 return 大约 50,000 条记录，但目前 return 不到 2000 条。

WITH FifthEnquiry AS
(
  SELECT emailaddress,
         SentDate,
         ROW_NUMBER() OVER (PARTITION BY emailaddress ORDER BY COUNT(*) DESC) AS rk
  FROM SentEmails
  GROUP BY emailaddress,
           SentDate
),
TenthEnquiry AS
(
  SELECT emailaddress,
         SentDate,
         ROW_NUMBER() OVER (PARTITION BY emailaddress ORDER BY COUNT(*) DESC) AS rk
  FROM SentEmails
  GROUP BY emailaddress,
           SentDate
),
TwentiethEnquiry AS
(
  SELECT emailaddress,
         SentDate,
         ROW_NUMBER() OVER (PARTITION BY emailaddress ORDER BY COUNT(*) DESC) AS rk
  FROM SentEmails
  GROUP BY emailaddress,
           SentDate
)
SELECT FifthEnquiry.emailaddress,
       FifthEnquiry.SentDate AS Fith,
       TenthEnquiry.SentDate AS Tenth,
       TwentiethEnquiry.SentDate AS Twentieth,
FROM FifthEnquiry
  JOIN TenthEnquiry ON FifthEnquiry.emailaddress = TenthEnquiry.emailaddress
  JOIN TwentiethEnquiry ON FifthEnquiry.emailaddress = TwentiethEnquiry.emailaddress
WHERE (FifthEnquiry.rk = 5)
AND   (TenthEnquiry.rk = 10)
AND   (TwentiethEnquiry.rk = 20)

Answer 1

你可以在很大程度上简化。并使用 LEFT JOIN 保留所有在 GROUP BY 之后至少有 5 行的电子邮件地址，即使没有第 10 或第 20 行：

WITH cte AS (
   SELECT emailaddress, SentDate,
          ROW_NUMBER() OVER (PARTITION BY emailaddress
                             ORDER BY COUNT(*) DESC, SentDate) AS rn
   FROM   SentEmails
   GROUP  BY 1,2
   )
SELECT enq05.emailaddress,
       enq05.SentDate AS fifth,
       enq10.SentDate AS tenth,
       enq20.SentDate AS twentieth
FROM        cte AS enq05
LEFT   JOIN cte AS enq10 ON enq10.emailaddress = enq05.emailaddress
                        AND enq10.rn = 10
LEFT   JOIN cte AS enq20 ON enq20.emailaddress = enq05.emailaddress
                        AND enq20.rn = 20
WHERE  enq05.rn = 5;

您不需要单独的 CTE，所有三个都在做同样的事情。 一个CTE就够了，而且速度明显更快。在外部查询中使用不同 table 别名的自连接。
因为我们现在使用LEFT JOIN，所以我们是否在 JOIN 或 WHERE 子句中添加附加条件很重要。 WHERE 子句中的条件有效地强制 Postgres 将连接视为普通 [INNER] JOIN。我相应地将条件移动到 JOIN 子句。详情：
- Join a count query on a generate_series in postgres and also retrieve Null-values as "0"
使用 rn，而不是 rk 作为列别名。这是 "row number"，而不是 "rank"。请注意在 row_number() and rank().
将 SentDate 添加到 ORDER BY 作为 (emailaddress, SentDate) 的决胜局，具有相同的计数以获得 stable 排序顺序。我的方式 SentDate IS NULL 每组排在最后。您可能希望使用 NULLS LAST 进行降序排序（不适用于 COUNT(*)，它永远不会为 NULL）：
- PostgreSQL sort by datetime asc, null first?
您需要注意一个更细微的细节：由于 两个不同的原因，如果 SentDate 可以在基础 table 中为 NULL。结果中 tenth 的 NULL 值可能意味着 emailaddress 的不同值少于 10 个，或者它可能意味着 NULL 根据您的排序顺序位于第 10 个位置。

Select With 语句中为 Null

Select Null in With Statement

sql

postgresql

select

null

common-table-expression