两列从一列读取

Question

我有这些列的表VISITS

Visit_ID , Location_ID , Next_Location_ID
----------
0001     , 101         , 102
0002     , 105         , 102
0003     , 102         , 105
----------

并且有一个表格，其中包含名称和其他信息的位置详细信息

Location
location_id , Name
----------
101         , US
102         , UK
.
.
.
105         , CA
----------

我的问题是如何编写SQL语句来检索这些信息

Visit_ID , Location_ID , Next_Location_ID
0001     , US          , UK
0002     , CA          , UK
0003     , UK          , CA

Answer 1

加入location表两次：

select v.Visit_ID, l1.Name, l2.Name
from VISITS v
join Location l1 on l1.location_id = v.Location_ID 
join Location l2 on l2.location_id = v.Next_Location_ID

Answer 2

将visits表加入location表两次 - 对visits中的每个外键加一次，如下所示：

select v.*, a.name location, b.name next_location
from visits v
join location a on a.location_id = v.location_id
join location b on b.location_id = v.next_location_id

执行此操作时，您必须使用表别名（此处我已使用＆＃34; a＆＃34;＆＃34; b＆＃34;）来区分在查询中互相乌鸦。