多对多关系的 3 个表之间的 SQL 查询

Question

我有三张表:friends , locations , friend_locationfriend_location是一个连接表，允许 friends 之间的多对多关系和 locations ，所以表格看起来像这样:

好友

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

地点

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

我想做的是获取每个 friend 的最新位置。

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按 location.id 分组，这会给你所有的位置，因为 ID 是唯一的。

这仅适用于 friend 在任何 1 次只能位于 1 个位置的情况。