Question

我有这个问题：

WITH grouped_messages AS
(SELECT
   um.conversation_id,
   array_agg(json_build_object('message', um.message, 'sent_at', um.created_at))
 FROM user_messages um
   INNER JOIN users_conversations c ON c.conversation_id = um.conversation_id
 WHERE c.user_id = '#{user_id}' AND um.user_id != '#{user_id}' #{ "and um.created_at >= '#{timestamp}'" IF TIMESTAMP}
 GROUP BY um.conversation_id
),
    senders AS
  (SELECT
     gm.conversation_id,
     json_build_object('id', u.id, 'user_name', u.user_name, 'avatar',
                       ('https://my-staging.s3.amazonaws.com/public/uploads/user/' || u.id :: TEXT ||
                        '/avatar.jpg')) AS sender
   FROM grouped_messages AS gm
     INNER JOIN users_conversations c ON c.conversation_id = gm.conversation_id
     INNER JOIN users u ON u.id = c.user_id
   WHERE u.id != '#{user_id}'
  )
SELECT json_object_agg(grouped_messages.conversation_id,
                       json_build_object('new_messages', grouped_messages.array_agg, 'sender', senders.sender))
FROM grouped_messages
  INNER JOIN senders ON senders.conversation_id = grouped_messages.conversation_id

哪些输出，例如：

{
    "62": {
        "new_messages": [
            {
                "message": "some",
                "sent_at": "2016-05-30T20:19:53.786024"
            },
            {
                "message": null,
                "sent_at": "2016-05-30T20:19:26.408814"
            }
        ],
        "sender": {
            "id": "e4ba308b-a5cf-47ad-b8d6-d774eb325411",
            "user_name": null,
            "avatar": "https://my-staging.s3.amazonaws.com/public/uploads/user/e4ba308b-a5cf-47ad-b8d6-d774eb325411/avatar.jpg"
        }
    }
}

现在，我需要的是在此查询中有一个条件语句。

如果u（用户）头像不为NULL，则应用此'https://my-staging.s3.amazonaws.com/public/uploads/user/' || u.id::text || '/avatar.jpg'，否则只需插入'：null'字符串（以便任何JSON解析器将其识别为空对象）。

不确定如何将这个if-else放在select语句中，以及如何确保该部分关于JSON解析器。

Answer 1

为我工作：

        (SELECT gm.conversation_id, json_build_object('id', u.id, 'user_name', u.user_name,
        'avatar', (
         CASE WHEN u.avatar IS NULL THEN ':null'
         ELSE ('https://my-staging.s3.amazonaws.com/public/uploads/user/' || u.id::text || '/avatar.jpg')
         END))

您可以在SELECT块中提供case语句，就好像它是值（带有可选AS）。

Answer 2

这是CASE表达式的一个很好的用法。在这种情况下，如果avatar字段不为空，您希望向其头像返回硬URL，否则返回NULL。

这样就完成了（为了便于阅读，省略了查询的其他部分）：

json_build_object('id', u.id, 'user_name', u.user_name,
                  'avatar',
                   CASE WHEN u.avatar IS NULL
                     THEN NULL
                   ELSE
                     (
                       'https://my-staging.s3.amazonaws.com/public/uploads/user/'
                       || u.id :: TEXT ||
                       '/avatar.jpg') END) AS sender

在PostgreSQL查询中添加条件语句

2 个答案: