Question

我正在做一些非常类似于这个SO线程（link）的事情。不同之处在于我的桌子有时会有空值。以下是我提供的链接后面的插图。

1）表'人'包含他们最喜欢的运动的人的列表

ID | Name  | FavSport | DislikeSport
------------------------------------
1  | Bob   |    1     |      5
2  | Roy   |    3     |      4
3  | Sarah |          |       
4  | Kevin |    2     |      
5  | Mary  |          |      3

2）表'运动'显示运动列表

ID |   Sport    
---------------
1  | Football   
2  | Basketball  
3  | Volleyball  
4  | Soccer    
5  | Badminton

我从我在此帖子中发布的链接中获得的查询

select people.Name, sports.Sport 
from people, sports 
where people.FavSport = sports.ID

3）如果查询类似于上面的结果

Name  | Sport      | Dislike
----------------------------
Bob   | Football   | Badminton 
Roy   | Volleyball | Soccer    
Sarah | Football   | Football   
Kevin | Basketball | Football   
Mary  | Football   | Volleyball

4）我想要的是如下所示的空（或任何默认值，如：没有）

Name  | Sport      | Dislike
----------------------------
Bob   | Football   | Badminton 
Roy   | Volleyball | Soccer    
Sarah |            | 
Kevin | Basketball | 
Mary  |            | Volleyball

有人如何处理这个问题？提前谢谢。

Answer 1

您需要外部联接

select people.Name,
       likes.Sport as FavSport ,
       dislikes.Sport as DislikeSport
from people
left join sports likes
on people.FavSport = likes.ID
left join sports dislikes
on people.DislikeSport = dislikes.ID

JOIN

的标准文档

Answer 2

SELECT people.NAME, 
       Isnull(sport.sport, '') AS Sport 
FROM   people 
       LEFT JOIN sport 
              ON favsport = sport.id

Answer 3

您需要执行left outer join。

select people.Name, sports.Sport 
  from people
  left join sports on (people.FavSport = sports.ID);

SQL：当有空值时，用字符串替换整数

3 个答案: