sql - 类别和子类别数据库

Question

我在检索列值时遇到问题，例如取决于其他列我尝试使用外部连接和嵌套选择来完成，但我想我无法在脑海中找到解决方案

Categories Table:
ID  Name                ParentID
--------------------------------
1   Software            NULL
2   Domains             NULL
3   Games               NULL
4   Accounts            NULL
5   Others              NULL
6   Security Software   1
7   Operating Systems   1
8   Browsers            1
9   Developer Tools     1
10  .com Domains        2
11  .net Domains        2
12  .org Domains        2
13  Online Games        3
14  PC Games            3
15  PS Games            3
16  RapidShare Accounts 4
17  4shared Account     4
18  Web Templates       5
19  Flash Intros        5
20  Firewall            6
21  Antivirus           6

我想要做的是显示父类别名称而不是 parentID

像那样

ID  Name                ParentID
--------------------------------
1   Software            NULL
2   Domains             NULL
3   Games               NULL
4   Accounts            NULL
5   Others              NULL
6   Security Software   Software
7   Operating Systems   Software
8   Browsers            Software
9   Developer Tools     Software
10  .com Domains        Domains
11  .net Domains        Domains
12  .org Domains        Domains
13  Online Games        Games               
14  PC Games            Games               
15  PS Games            Games               
16  RapidShare Accounts Accounts            
17  4shared Account     Accounts            
18  Web Templates       Others              
19  Flash Intros        Others              
20  Firewall            Security Software
21  Antivirus           Security Software

Answer 1

你需要在桌子上加入两次

SELECT c1.Id, c1.Name as parentname , c2.name
FROM categories c1
JOIN categories c2
    ON c1.id = c2.parentid

结果将是：

ID      ParentName            Name
1       software              security software
1       software              operating software
2       domains               .com domains
2       domains               .net domains
2       domains               .org domains
3       games                 pc games

这将为您提供子类别或子类别的 parentID、parentName 和 Name。

如果需要，您还可以将其更改为包含子类别 ID。

根据您的编辑，您可以使用以下内容：

create table categories
(
    id int,
    name varchar(50),
    parentid int
)

insert into categories values(1, 'software', null)
insert into categories values(2, 'domains', null)
insert into categories values(3, 'games', null)
insert into categories values(6, 'security software', 1)
insert into categories values(7, 'operating systems', 1)
insert into categories values(8, 'browsers', 1)
insert into categories values(10, '.com domains', 2)
insert into categories values(11, '.net domains', 2)
insert into categories values(12, '.org domains', 2)
insert into categories values(13, 'online games', 3)
insert into categories values(14, 'pc games', 3)
insert into categories values(15, 'ps games', 3)

select c1.id
    , c1.name
    , (select name from categories where c1.parentid = categories.id) as ParentId
from categories c1

这将导致：