SUM（some_column）WHERE some_column匹配(SUM (some_column) WHERE some_column matches)

 所以我有一个表，我正在尝试获取SUM（activity_weight）WHERE activity_typeid是唯一的。 
 
 每场比赛都有无限量的activity_typeid。 
 
 正如您在下面的代码中看到的，我想知道是否有一些SQL函数可以找到某些内容的SUM，例如id是唯一的？ 
 
 感谢任何提前帮助！ 
 
 我在下面附上了我的桌子和所需输出的照片 
 
 
 SELECT a.userid, u.name, u.profilePic , 
  SUM(activity_weight) AS totalPoints , 

  //sum the points when the activity_typeid is unique and make an extra column for each of those sums
  SUM(CASE WHEN activity_typeid //is unique// THEN activity_weight    ELSE NULL END) AS specific_points , 

  FROM activity_entries a INNER JOIN users1 u ON u.id = a.userid 

  WHERE competitionId = '$competitionId' GROUP BY a.userid ORDER BY totalPoints

So I have a table, and I'm trying to get the SUM(activity_weight) WHERE activity_typeid is unique.
 
Each competition has an unlimited amount of activity_typeid's.
 
As you can see in my code below, I wonder if there is some SQL function to find the SUM of something WHERE the id is unique for example?
 
THANKS FOR ANY HELP IN ADVANCE!
 
I've attached a photo of my table and desired output below
 
 
 SELECT a.userid, u.name, u.profilePic , 
  SUM(activity_weight) AS totalPoints , 

  //sum the points when the activity_typeid is unique and make an extra column for each of those sums
  SUM(CASE WHEN activity_typeid //is unique// THEN activity_weight    ELSE NULL END) AS specific_points , 

  FROM activity_entries a INNER JOIN users1 u ON u.id = a.userid 

  WHERE competitionId = '$competitionId' GROUP BY a.userid ORDER BY totalPoints

原文：https://stackoverflow.com/questions/18362322