结合SQL连接和计数

我有两个表

registered

和

attended

，每个表都有两列：

AttendentId

和

SessionId

。我想分别从这两个表中查询特定会话id的

AttendantId

计数

例如：

登记 出席 我想获得以下输出：

Count(Registered)   Count(Attended)  Session ID    
2                   1                SN1    
1                   0                SN2 
1                   1                SN3

---

您可以使用

完全外部连接

：

select 
  coalesce(a.sessionid, r.sessionid) sessionid,
  count(r.AttendantId) countRegistered,
  count(a.AttendantId) countAttended
from registered r
full outer join attended a
  on r.sessionid = a.sessionid
  and r.AttendantId = a.AttendantId
group by coalesce(a.sessionid, r.sessionid);

---

请参见您可以使用

完全外部联接
：

select 
  coalesce(a.sessionid, r.sessionid) sessionid,
  count(r.AttendantId) countRegistered,
  count(a.AttendantId) countAttended
from registered r
full outer join attended a
  on r.sessionid = a.sessionid
  and r.AttendantId = a.AttendantId
group by coalesce(a.sessionid, r.sessionid);

请参见尝试：

select count(distinct registered), 
       count(distinct attended), 
       SessionId
from (select AttendantId registered, null attended, SessionId
      from registered
      union all
      select null registered, AttendantId attended, SessionId
      from Attended) sq
group by SessionId

尝试：

请尝试此查询

select 
   a.sessionId, 
   case when aCnt is null then 0 else acnt end, 
   case when bCnt is null then 0 else bcnt end
from
   (select 
       sessionId, 
       count(*) aCNt 
   from 
       tbl1 
   group by 
       sessionid) a
full join
   (select 
       sessionId, 
       count(*) bCnt 
   from 
       tbl2 
   group by 
       sessionid) b
on 
   a.sessionId = b.sessionid

：
希望这有助于……
尝试此查询

select 
   a.sessionId, 
   case when aCnt is null then 0 else acnt end, 
   case when bCnt is null then 0 else bcnt end
from
   (select 
       sessionId, 
       count(*) aCNt 
   from 
       tbl1 
   group by 
       sessionid) a
full join
   (select 
       sessionId, 
       count(*) bCnt 
   from 
       tbl2 
   group by 
       sessionid) b
on 
   a.sessionId = b.sessionid

：
希望这有助于……
假设已注册表中存在所有会话ID

SELECT Sum(CASE 
             WHEN a.attendentid IS NOT NULL THEN 1 
             ELSE 0 
           end) AS Count(registered), 
       Sum(CASE 
             WHEN b.attendentid IS NOT NULL THEN 1 
             ELSE 0 
           end) AS Count(attended), 
       a.sessionid 
FROM   registered a 
       INNER JOIN attended b 
               ON a.sessionid = b.sessionid 
GROUP  BY a.sessionid 

假设已注册表中存在所有会话ID

SELECT Sum(CASE 
             WHEN a.attendentid IS NOT NULL THEN 1 
             ELSE 0 
           end) AS Count(registered), 
       Sum(CASE 
             WHEN b.attendentid IS NOT NULL THEN 1 
             ELSE 0 
           end) AS Count(attended), 
       a.sessionid 
FROM   registered a 
       INNER JOIN attended b 
               ON a.sessionid = b.sessionid 
GROUP  BY a.sessionid 

到目前为止你试过什么？请添加到您的问题中，以便我们看到您的错误所在。示例输出中前两列的标题混淆了。我尝试使用基于会话ID的联接，但它返回的是助理ID的叉积。registered中的会话ID与attended中的会话ID之间的关系是什么？参加的会话ID是否始终是已注册会话ID的子集？到目前为止，您尝试了什么？请添加到您的问题中，以便我们看到您的错误所在。示例输出中前两列的标题混淆了。我尝试使用基于会话ID的联接，但它返回的是助理ID的叉积。registered中的会话ID与attended中的会话ID之间的关系是什么？出席的会话ID是否始终是已注册会话ID的子集？@user1989463:很高兴我能提供帮助。@user1989463:很高兴我能提供帮助。
SELECT Sum(CASE 
             WHEN a.attendentid IS NOT NULL THEN 1 
             ELSE 0 
           end) AS Count(registered), 
       Sum(CASE 
             WHEN b.attendentid IS NOT NULL THEN 1 
             ELSE 0 
           end) AS Count(attended), 
       a.sessionid 
FROM   registered a 
       INNER JOIN attended b 
               ON a.sessionid = b.sessionid 
GROUP  BY a.sessionid