Question

我的问题在于第一个子查询选择。我被告知我要返回多行。

$sql = "SELECT
      messages.message_id
    , messages.sent_timestamp
    , messages.content
    , messages.subject
    , users.user_name
    , (SELECT thread_participants.user_id 
FROM thread_participants 
WHERE thread_participants.user_id !=".$user_id.") as thread_participants
    , (SELECT message_read_state.readDate
       FROM message_read_state
       WHERE message_read_state.message_id = messages.message_id
        and message_read_state.user_id =". $user_id.") as ReadState
    FROM (messages INNER JOIN users ON messages.sender_user_id = users.user_id
    INNER JOIN thread_participants tp ON tp.thread_id = messages.thread_id)
    WHERE (((messages.thread_id)=".$thread_id."))
    ORDER BY messages.sent_timestamp DESC";

Answer 1

您可以使用GROUP_CONCAT()功能在单个字段中获取所有 userIds

试试这个：

$SQL = "SELECT
      messages.message_id
    , messages.sent_timestamp
    , messages.content
    , messages.subject
    , users.user_name
    , (SELECT GROUP_CONCAT(thread_participants.user_id) 
FROM thread_participants 
WHERE thread_participants.user_id !=".$user_id.") as thread_participants
    , (SELECT message_read_state.readDate
       FROM message_read_state
       WHERE message_read_state.message_id = messages.message_id
        and message_read_state.user_id =". $user_id.") as ReadState
    FROM (messages INNER JOIN users ON messages.sender_user_id = users.user_id
    INNER JOIN thread_participants tp ON tp.thread_id = messages.thread_id)
    WHERE (((messages.thread_id)=".$thread_id."))
    ORDER BY messages.sent_timestamp DESC";

Answer 2

在大多数情况下，在mysql中，子查询不如使用单独的查询更佳。在这种情况下，最好使用先前查询中的数据（如id的集合）并在另一个查询中使用它。对您的数据进行一些测试。通常数据量会产生差异;）

同样，分割查询可以帮助您更好地缓存数据。

Answer 3

我发现这样做的一种方式：

$sql = "SELECT
      messages.message_id
    , messages.sent_timestamp
    , messages.content
    , messages.subject
    , users.user_name
    , tp.user_id as thread_participants
    , (SELECT users.user_name FROM users WHERE users.user_id = thread_participants && users.user_id != messages.sender_user_id) as member_names
    , (SELECT message_read_state.readDate
       FROM message_read_state
       WHERE message_read_state.message_id = messages.message_id
        and message_read_state.user_id =". $user_id.") as ReadState
    FROM (messages INNER JOIN users ON messages.sender_user_id = users.user_id
    INNER JOIN thread_participants tp ON tp.thread_id = messages.thread_id)
    WHERE (((messages.thread_id)=".$thread_id."))
    ORDER BY messages.sent_timestamp DESC";

mysql子查询返回多行

3 个答案: