Question

我有三个表：friends，locations，friend_location

friend_location是一个连接表，允许friends和locations之间的多对多关系，因此表格看起来像这样：

友


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

位置


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

我想做的是为每位朋友获取最新位置。

结果


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

这是我在查看各种示例后尝试过的，但它返回了许多结果并且不正确：


    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

任何建议都将不胜感激。

Answer 1

你可以这样做：

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

基本上你的问题是你是按location.id进行分组，这会给你所有的位置，因为ID是唯一的。

这仅适用于朋友一次只能在1个位置的情况。

Answer 2

Gregs查询对我来说是正确的，我想出了类似的（见下文）。但是，当两个朋友以不同顺序访问相同位置时，当前的db模式不处理这种情况。对我来说，似乎Date列应该在friend_location表中，而不是位置。但是，如果不是，那么查询是：

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

您的数据布局有点奇怪，因为日期位于位置表中。因此，以下内容检索每位朋友的最新日期：

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

现在，让我们重新加入此查询的信息：

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

这将起作用，假设位置没有重复日期。如果他们这样做，查询会更复杂一些。我们可以做出这个假设吗？

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

在多对多关系的3个表之间的SQL查询

4 个答案: