多對多關系的3個表之間的SQL查詢

Question

我有三個表： friends ， locations ， friend_location

friend_location是一個friend_location表，用於允許friends和locations之間進行多對多關系，因此這些表將如下所示：

友人

ID  | Name
1   | Jerry
2   | Nelson
3   | Paul

地點

ID  | Date       | Lat   | Lon 
1   | 2012-03-01 | 34.3  |  67.3
2   | 2011-04-03 | 45.3  |  49.3
3   | 2012-05-03 | 32.2  |  107.2

friend_location

Friend_ID  | Location_id
1          |  2
2          |  1
3          |  3
2          |  2

我想做的就是獲取每個朋友的最新位置。

結果

ID  | Friend | Last Know Location  | last know date
1   | Jerry  |  45.3 , 49.3        | 2011-04-03
2   | Nelson |  34.3 , 67.3        | 2012-03-01
3   | Paul   |  32.2 , 107.2       | 2012-05-03

這是我在查看各種示例之后嘗試過的方法，但是它返回了許多結果，並且是不正確的：

    select f.id , f.name , last_known_date
    from friends f, (

    select distinct fl.friend_id as friend_id, fl.location_id as location_id, m.date as last_known_date
    from friend_location fl

    inner join (
        select location.id as id, max(date) as date
        from location
        group by location.id
    ) m

    on fl.location_id=m.id

    ) as y
    where f.id=y.friend_id

任何建議將不勝感激。

Answer 1

您可以執行以下操作：

SELECT  f.id, f.name, last_known_date, l.Lat, L.Lon
from Friends f
join
(
    select  f.id, MAX(l.Date) as last_known_date
    from    Friends f
    JOIN    Friend_Location fl on f.ID = fl.Friend_ID
    JOIN    Location l on l.ID = fl.Location_ID
    GROUP BY f.id
) FLMax
on FLMax.id = f.id
join Friend_Location fl on fl.friend_ID = f.ID
join Location l on fl.location_ID = l.ID AND l.Date = FLMax.Last_Known_Date

基本上，您的問題是您要按location.id分組，因為ID是唯一的，所以它將為您提供所有位置。

僅當朋友在任何一次只能位於1個位置時，此方法才有效。

Answer 2

Gregs查詢對我來說看起來是正確的，我想出了類似的一個（見下文）。 但是，當兩個朋友以不同順序訪問相同位置時，當前的數據庫架構無法處理這種情況。 對我來說，日期列似乎應該在friend_location表中，而不是location中。 但是，如果不是，則查詢為：

SELECT F.ID, F.Name AS Friend, L.Lat, L.Lon, L.Date
FROM
(
    SELECT MAX(L.date) AS max_date, F.ID
    FROM Friends F 
    JOIN friend_location FL ON F.ID=FL.Friend_ID
    JOIN Location L ON L.ID=FL.Location_id
    GROUP BY F.ID
) AS X
JOIN Friends F ON X.ID=F.ID
JOIN friend_location FL ON F.ID=FL.Friend_ID
JOIN location L ON L.ID=FL.Location_id AND L.Date=X.max_date

Answer 3

您的數據布局有點奇怪，因為日期在位置表中。 因此，以下將檢索每個朋友的最新日期：

select fl.friend_id, max(l.date) as maxdate
from friend_location fl
     location l join
     on fl.location_id = l.location_id

現在，讓我們將信息加入此查詢中：

select f.*, maxdate, l.*
from (select fl.friend_id, max(l.date) as maxdate
      from friend_location fl
      JOIN location l
           on fl.location_id = l.id
      group by fl.friend_id
     ) flmax join
     friends f
       on flmax.friend_id = f.id
     join location l
       on l.date = flmax.maxdate

假設這些地點沒有重復的日期，這將起作用。 如果這樣做的話，查詢會更加復雜。 我們可以做這個假設嗎？

Answer 4

您可以使用：

SELECT 
    a.*,
    CONCAT(d.Lat, ' , ', d.Lon) AS last_known_location,
    d.Date AS last_known_date
FROM 
    friends a
JOIN
(
    SELECT   a.Friend_ID, MAX(b.Date) AS maxdate
    FROM     friend_location a
    JOIN     location b ON a.Location_id = b.ID
    GROUP BY a.Friend_ID
) b ON a.ID = b.Friend_ID
JOIN
    friend_location c ON b.Friend_ID = c.Friend_ID
JOIN
    location d ON c.Location_id = d.ID AND b.maxdate = d.Date