Question

我正在将一些SQL逻辑从SSMS中使用的T-SQL转换为Amazon Redshift。 I believe Redshift is a fork of Postgres version 8.0.2所以除非使用Postgres 9.1，否则以下内容可能无法实现。

WITH CTE_ID AS 
(
SELECT FULL_NAME, COUNT(DISTINCT ID) as ID_COUNT, MAX(ID) AS MAX_ID
FROM MEMBERS
GROUP BY FULL_NAME
HAVING COUNT(DISTINCT ID) > 1
)
UPDATE a
SET a.ID = b.MAX_ID
FROM MEMBERS a
INNER JOIN CTE_ID b
ON a.FULL_NAME = b.FULL_NAME

如果Amazon Redshift不支持此功能，我最好的选择是创建一个新的“临时”表并用CTE生成的值填充它吗？

Answer 1

您可以将查询重写为派生表，如@a_horse_with_no_name所述：

UPDATE a
SET a.ID = b.MAX_ID
FROM MEMBERS a
INNER JOIN (
  SELECT FULL_NAME, COUNT(DISTINCT ID) as ID_COUNT, MAX(ID) AS MAX_ID
  FROM MEMBERS
  GROUP BY FULL_NAME
  HAVING COUNT(DISTINCT ID) > 1
  ) b
ON a.FULL_NAME = b.FULL_NAME

Answer 2

创建一个临时表而不是with子句将起作用。

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Answer 3

现有答案（包括已接受的答案）无效。这应该有效：

UPDATE members AS a
SET    id = b.max_id
FROM  (
   SELECT full_name, max(id) AS max_id
   FROM   members
   GROUP  BY full_name
   HAVING count(DISTINCT id) > 1
   ) b
WHERE  a.full_name = b.full_name
AND    a.id IS DISTINCT FROM b.max_id;

不需要 CTE（尽管可能）。子查询更简单。

目标表只列出一次。您只需在 FROM 子句中使用（不同的）别名重复它以满足特殊需要。

SET 列表中的目标列不能是表限定的。

未加引号的 names are folded to lower case in Redshift。大写拼写只会增加混乱。

我添加了谓词 AND a.id IS DISTINCT FROM b.max_id 以跳过不会更改的行的更新。（昂贵的无操作。）您只希望那些在特殊情况下触发（未声明的）副作用。

Redshift manual for UPDATE 中的更多内容。

在Redshift中使用CTE和Update

3 个答案: