sql - 在多个表中递归的 Sql server 查询

Question

我的表结构如下，示例数据如下名称：用户

User   Region
Sam    USA    
Meyer  NA    
Ying   Japan    
Yang   Asia

我还有另一张桌子，上面有地区和国家

名称：地区

       Region Country         RegionList    
        USA     America            
        NA       NULL          USA            
        NA      Canada            
        NA      Mexico            
        Japan   Japan
        Asia     NULL         Central        
        Asia      NULL         East        
        Central  India        
        East     Dubai
        Central  Japan

你如何编写一个 SQL 服务器查询来列出他/她代表的每个用户和相应的国家？

像：

Sam   America
Meyer America
Meyer Canda
Meyer Mexico
Ying  Japan
Yang  Japan
Yang  India
Yang  Dubai

Answer 1

如果您的 Region 表可能有无限多个级别，则可以使用Common Table Expression。请参阅使用公用表表达式的递归查询。

WITH UserRegions (username, region, country)
AS
(
    -- Anchor member definition
    SELECT u.[user] AS username, r.regionlist AS region, r.country
    FROM [User] u
    JOIN Region r
        ON u.Region = r.Region
    UNION ALL
    -- Recursive member definition
    SELECT ur.username, r.regionlist AS region, r.country
    FROM Region r
    INNER JOIN UserRegions AS ur
        ON r.Region = ur.Region
)
-- Statement that executes the CTE
SELECT username, country
FROM UserRegions
WHERE country IS NOT NULL
ORDER BY username

http://sqlfiddle.com/#!3/ca9de/8

Answer 2

您可以使用 JOIN 来获得您想要的结果

SELECT user, isNull(Country,RegionList) Country
FROM user
    JOIN region ON users.region = region.region
ORDER BY user, Country

如果您想返回所有用户，即使他们没有任何国家，那么您可以使用 LEFT OUTER JOIN

SELECT user, isNull(isNull(Country,RegionList),'No countries') Country
FROM user
    LEFT OUTER JOIN region ON users.region = region.region
ORDER BY user, Country

Answer 3

根据您提供的显式架构和示例数据：

select
user, isnull(region.country, r2.country) as Country 
from 
user
left join 
region 
on user.region = region.region
left join
region r2
on r2.region = region.regionlist