sql - 将 SQL 连接与计数相结合

Question

我有两个表registeredand attended，每个表有两列：AttendentIdand SessionId。我想分别AttendantId从这两个表中查询特定会话 ID 的计数。

例子：

挂号的

AttendantId      SessionId    
ID1              SN1    
ID2              SN2
ID3              SN1
ID4              SN3

出席

AttendantId       SessionId    
ID1               SN1   
ID4               SN3

我想获得以下输出：

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

Answer 1

您可以使用FULL OUTER JOIN：

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);

请参阅带有演示的 SQL Fiddle

Answer 2

SELECT
    ISNULL(ACount, 0),
    ISNULL(RCount, 0),
    X.SessionId
FROM
    (
    SELECT SessionId FROM Registered
    UNION -- implies DISTINCT
    SELECT SessionId FROM Attended
    ) X
    LEFT JOIN
    (SELECT COUNT(*) AS RCount, SessionId
     FROM Registered
     GROUP BY SessionId) R ON X.SessionId = R.SessionId
    LEFT JOIN
    (SELECT COUNT(*) AS ACount, SessionId
     FROM Attended 
     GROUP BY SessionId) A ON X.SessionId = A.SessionId

SQLFiddle

Answer 3

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

Answer 4

试试这个查询

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

SQL 小提琴：

| SESSIONID | COLUMN_1 | COLUMN_2 |
-----------------------------------
|       SN1 |        2 |        1 |
|       SN2 |        1 |        0 |
|       SN3 |        1 |        1 |

希望这可以帮助....

Answer 5

假设所有会话 ID 都存在于注册表中

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