加入SQL表

Question

我有兩個表：

RecommendedFriends和AddedFriends

每個表中有一個用戶字段和朋友現場。我想弄清楚我怎麼能看到有多少朋友加入了他們也被推薦。繼承人的表的例子：

RecommendedFriends 
User Friends  Time 
------------------------------------ 
Jake Eric  8:00am 
Jake John  8:00am 
Jake Jack  8:30am 
Greg John  8:30am 
Greg Tim   9:00am 
Greg Steve  9:30am 
Will Jackson  9:30am 

AddedFriends 
User Friends  Time 
------------------------------------ 
Jake Jack  8:35am 
Greg John  8:35am 
Greg Tim   9:00pm 
Greg Jim   10:30am 
Greg Tina  10:45am 
Greg Bob   10:00am 
Charlie Brian  11:00am 

所以我需要看起來像這樣的表：

Results 
User RecFriends AddFriends 
------------------------------------ 
Jake Eric   
Jake John   
Jake Jack  Jack 
Greg John  John 
Greg Tim   Tim 
Greg Steve  
Greg    Tina 
Will Jackson  
Charlie    Brian 

所以我可以去，說3人加入，他們建議的朋友，4項建議失敗，和2人添加了他們不被推薦的人。

Answer 1

我想你想要的是full outer join：

select coalesce(rf.USER, af.user) as user, rf.friends as RecFriends, af.Friends as AddFriends, 
from RecommendedFriends rf full outer join 
    AddedFriends af 
    on rf.user = af.user and 
     rf.Friends = af.Friends 

這並不需要時間考慮。如果您想要推斷推薦和添加之間的因果關係，您可能需要檢查添加的時間是在推薦時間之後。

如果您使用的是不支持full outer join（任何人都可以說「MySQL的」）的數據庫，你可以得到相同的結果做：

select t.user, MAX(case when which = 'rec' then friends end) as RecFriends, 
     MAX(case when which = 'add' then friends end) as AddFriends 
from ((select rf.user, rf.friends, 'rec' as which 
     from RecommendedFriends af.user 
    ) union all 
     (select af.user, af.friends, 'add' as which 
     from AddedFriends af 
    ) 
    ) t 
group by user 

該版本具有很好的功能，它不會產生重複的記錄，如果有多個建議或增加。