在sql中加入两个表并过滤记录

Question

我需要在Room No上加入两个表值。

这是我的表格：

RoomStatusTable

----------------------------------------------------
| Room No   |   Type   |  Date     |  Descriprion  |
----------------------------------------------------
|  1        |    AC    |22 Jan 2015|   VA          |
|  2        |    AC    |22 Jan 2015|   VA          |
|  3        |    AC    |22 Jan 2015|   VA          |
|  4        |    AC    |22 Jan 2015|   VA          |

RoomBooked表

----------------------------------------------------
| Room No   |   Type   |  Date     |  Descriprion  |
----------------------------------------------------
|  1        |    AC    |22 Jan 2015|   BK001       |
|  2        |    AC    |22 Jan 2015|   BLOCK       |
|  4        |    AC    |22 Jan 2015|   OC001       |

我需要一个如下结果：

----------------------------------------------------
| Room No   |   Type   |  Date     |  Descriprion  |
----------------------------------------------------
|  1        |    AC    |22 Jan 2015|   BK001       |
|  2        |    AC    |22 Jan 2015|   Block       |
|  3        |    AC    |22 Jan 2015|   VA          |
|  4        |    AC    |22 Jan 2015|   OC001       |

与此类似，表1包含所有记录，但是，表2仅包含基于该更改描述的占用记录作为我的结果。

Answer 1

使用Left join从两个表中获取数据：

select r1.room_no,
       r1.Type,
       r1.Date, 
case when r2.Descriprion is null then r1.Descriprion else r2.Descriprion end as description   
from RoomStatus r1 
left join RoomBooked r2 on r1.room_no=r2.room_no
order by r1.room_no;

Answer 2

使用Coalesce函数返回两个描述列之间的第一个非null值，

select r1.room_no,
       r1.Type,
       r1.Date, 
       coalesce(r2.Descriprion ,r1.Descriprion) As description   
from RoomStatus r1 
left join RoomBooked r2 on r1.room_no = r2.room_no
order by r1.room_no;