Postgres 区分大小写

Question

我有一个查询，我在 DISTINCT ON 中使用 CASE 语句，如下所示：

SELECT
    DISTINCT ON(COALESCE(users.cuid,events.uid),
        CASE WHEN events.props->>'prop' IS NULL THEN '$none' WHEN events.props->>'prop' = '' THEN '$none' ELSE events.props->>'prop' END)
    COALESCE(users.cuid, events.uid) as coal_user_id,
    CASE WHEN events.props->>'prop' IS NULL THEN '$none' WHEN events.props->>'prop' = '' THEN '$none' ELSE events.props->>'prop' END AS _group_key_0, events.uid as event_uid
FROM events JOIN users ON events.uid=users.id 
WHERE events.project_id='<>' AND timestamp>='<>' AND timestamp<='<>'
ORDER BY coal_user_id, _group_key_0, events.timestamp ASC

在运行，我收到此错误：

SELECT DISTINCT ON expressions must match initial ORDER BY expressions

我尝试在 distinct on 子句中使用像 as _group_key_0 这样的别名，但这又是一个不同的错误。

有没有办法在 distinct on 中使用 CASE 语句并使其工作？

Answer 1

您必须在 ORDER BY 子句的开头添加 DISTINCT ON 子句中的表达式，如错误消息所述：

SELECT DISTINCT ON(COALESCE(users.cuid,events.uid),
                   CASE WHEN events.props->>'prop' IS NULL THEN '$none'
                        WHEN events.props->>'prop' = '' THEN '$none'
                        ELSE events.props->>'prop'
                   END)
       COALESCE(users.cuid, events.uid) as coal_user_id,
       CASE WHEN events.props->>'prop' IS NULL THEN '$none'
            WHEN events.props->>'prop' = '' THEN '$none'
            ELSE events.props->>'prop'
       END AS _group_key_0,
       events.uid as event_uid
FROM events JOIN users ON events.uid=users.id 
WHERE events.project_id='<>'
  AND timestamp>='<>'
  AND timestamp<='<>'
ORDER BY COALESCE(users.cuid,events.uid),
         CASE WHEN events.props->>'prop' IS NULL THEN '$none'
              WHEN events.props->>'prop' = '' THEN '$none'
              ELSE events.props->>'prop'
         END,
         events.timestamp ASC;

Postgres 区分大小写

Postgres distinct on with case

postgresql

distinct-on