Question

我有以下两个名为ROOM和SESSION的表。

ROOM

BUILDINGNO | ROOMNO | ROOMCAPACITY
-----------+--------+-------------
B1         | R1     | 121
B1         | R2     | 253
B2         | R1     | 275
B2         | R2     | 126
B2         | R3     | 124
B3         | R1     | 256    
B3         | R2     | 324
B4         | R1     | 431
B4         | R2     | 345
B4         | R3     | 231


SESSION

SESSIONID | BUILDINGNO | ROOMNO | SPEAKERID
----------+------------+--------+----------
SS01      | B1         | R1     | S1
SS02      | B2         | R1     | S2
SS03      | B1         | R2     | S2
SS04      | B4         | R2     | S4
SS05      | B3         | R2     | S5
SS06      | B3         | R1     | S5
SS07      | B4         | R2     | S3
SS08      | B1         | R2     | S2
SS09      | B2         | R3     | S4
SS10      | B4         | R1     | S3

我希望能够显示每个房间的会话数量。理想情况下，这会显示出某种类似的表格。

BUILDINGNO | ROOMNO | NUMBEROFSESSIONS
-----------+--------+-----------------
B1         | R1     | 1
B1         | R2     | 2
B2         | R1     | 1
B2         | R2     | 0
B2         | R3     | 1
B3         | R1     | 1  
B3         | R2     | 1
B4         | R1     | 1
B4         | R2     | 2
B4         | R3     | 0

我知道我需要在某个地方使用COUNT和JOIN功能......但我至少可以说有点卡住了。如果有人能指出我正确的方向或向我解释，我将不胜感激：）

SELECT BUILDINGNUMBER, ROOMNO FROM ROOM ... COUNT? ... JOIN?

还有什么可以让我显示/隐藏会话数= 0的行？

Answer 1

如果您只对每个会议室内的会话数感兴趣，那么我相信您在 SESSION 表中有足够的信息。

SELECT BUILDINGNO, ROOMNO, COUNT(*)FROM SESSION 
GROUP BY BUILDINGNO, ROOMNO;

Here is a sql fiddle:

Answer 2

You could use count(distinct sessionid)

SELECT ROOM.BUILDINGNO , ROOM.ROOMNO, count(distinct SESSION.sessionid)
from ROOM 
left join SESSION on ROOM.ROOMNO = SESSION.ROOMNO 
     and ROOM.BUILDINGNO = SESSION.BUILDINGNO 
group by  ROOM.BUILDINGNUMBER, ROOM.ROOMNO

and for hide room with resul = 0 you can use having

SELECT ROOM.BUILDINGNO , ROOM.ROOMNO, count(distinct SESSION.sessionid)
from ROOM 
left join SESSION on ROOM.ROOMNO = SESSION.ROOMNO 
     and ROOM.BUILDINGNO = SESSION.BUILDINGNO 
group by  ROOM.BUILDINGNUMBER, ROOM.ROOMNO
HAVING count(distinct SESSION.sessionid) >0

but you have enough info in the tables sessions

SELECT SESSION.BUILDINGNO , SESSION.ROOMNO, count(distinct SESSION.sessionid)
SESSION 
group by  SESSION.BUILDINGNO , SESSION.ROOMNO
HAVING count(distinct SESSION.sessionid) >0

Answer 3

SELECT r.BUILDINGNO, r.ROOMNO, count(1) QQ
FROM ROOM r, SESS s
WHERE 
r.ROOMNO = s.ROOMNO
AND r.BUILDINGNO = s.BUILDINGNO
GROUP BY r.BUILDINGNO, r.ROOMNO
ORDER by r.BUILDINGNO;

[DEMO] [1] http://sqlfiddle.com/#!4/eb290/6

如何获取每个项目的计数

3 个答案: