mysql - 如何对具有不同值但相同名称的主键进行分组并使用 datediff

Question

所以我有三个表:

客人可以进行预订，在预订表中您可以根据主键“pID”查看客人进行的预订类型。我只想显示豪华客房的预订以及客人在那里停留的天数。我什至不知道我正在做的事情是正确的。我希望有人能帮助我。

预订:

pID              |begindate    | enddate     |   
------------------------------------------------------
COD12            | 2014-07-15  | 2014-07-18  |
COD400           | 2014-07-20  | 2014-07-21  |
KOD12            | 2014-07-01  | 2014-07-07  |

豪华客房 table :

pID              |city         |    
---------------------------------
COD12            | Corona      | 
COD400           | Corona      | 
KOD12            | Kentucky    | 

Small room table:

pID              |city         |    
---------------------------------
COD600           | Corona      |
MOD10            | Madrid      |
KOD20            | Kentucky    | 

我想要什么: 客人在豪华客房停留的天数，请注意 Corona 有两个房间:

city             |amountofdays |    
---------------------------------
Corona           | 4           |
Kentucky         | 6           |

我尝试做的事情:

SELECT datediff(reservation.enddate,reservation.begindate) as amountofdays, luxeroom.city FROM reservation
INNER JOIN luxeroom ON reservation.pID = luxeroom.pID
GROUP BY luxeroom.pID, luxeroom.city; 

this resulted in:

city             |amountofdays |    
---------------------------------
Corona           | 3           |
Corona           | 1           |
Kentucky         | 6           |

Answer 1

您需要分组依据和求和

SELECT sum(datediff(a.enddate, a.begindate)) as amountofdays, b.city
FROM reservation a 
JOIN luxeroom b  ON a.pID = b.pID
GROUP BY b.city;