如果记录存在则使用内连接,否则使用左连接

Question

我有以下表结构:

dbo.Owner

OwnerID   OwnerName  
  1        John   
  2        Marie
  3        Alex

和dbo.Pet

PetID PetTag Status OwnerID
  1    A341  Active    1  
  2    A342  Inactive  1  
  3    A343  Active    2
  4    A345  Active    2

我需要归还所有只有活动宠物或没有宠物的主人.

所以在上面这个例子中,我需要返回所有者2(所有宠物都有效)和所有者3(没有宠物)

我将使用Entity Framework在C#中提取数据,但是纯SQL就足够了.

这是我到目前为止所提出的:

select mi.* from Owner o
join Pet p
on o.OwnerID= p.OwnerID
where o.Status='Active'
union select * from Owner
where OwnerID not in (select OwnerID from Pet)

现在,上面的这个查询有效但它包含OwnerID = 1.而且我想知道是否有一种方法可以在没有union的1个查询中执行此操作.

Answer 1

如果您的唯一值为Status"活动"和"非活动",则实际上可以简化查询.当你说:

我需要归还所有只有活动宠物或没有宠物的主人.

这将实际转化为:

我需要归还所有没有非活动宠物的主人.

然后您的查询变得更容易.

在实体框架查询中:

owners = context.Owners
    .Where(o => !o.Pets.Any(p => p.Status == "Inactive"))
    .ToList();

由此生成的SQL查询是:

SELECT 
    [Extent1].[OwnerID] AS [OwnerID], 
    [Extent1].[OwnerName] AS [OwnerName]
    FROM [dbo].[Owners] AS [Extent1]
    WHERE  NOT EXISTS (SELECT 
        1 AS [C1]
        FROM [dbo].[Pets] AS [Extent2]
        WHERE ([Extent1].[OwnerID] = [Extent2].[OwnerID]) AND (N'Inactive' = [Extent2].[Status])
    )

或者删除杂乱:

SELECT 
    OwnerID,
    OwnerName
    FROM Owners o
    WHERE  NOT EXISTS (SELECT 
        1
        FROM Pets p
        WHERE (o.OwnerID = p.OwnerID AND p.Status = 'Inactive')
    )

如果您有更多Status的值,您可以使用(Entity Framework):

owners = context.Owners
    .Where(o => o.Pets.Any(p => p.Status == "Active") || !o.Pets.Any())
    .Where(o => !o.Pets.Any(p => p.Status == "Inactive" /* || p.Status == "Lost" and any other values */))
    .ToList();

这将生成SQL查询:

SELECT 
    [Extent1].[OwnerID] AS [OwnerID], 
    [Extent1].[OwnerName] AS [OwnerName]
    FROM [dbo].[Owners] AS [Extent1]
    WHERE (( EXISTS (SELECT 
        1 AS [C1]
        FROM [dbo].[Pets] AS [Extent2]
        WHERE ([Extent1].[OwnerID] = [Extent2].[OwnerID]) AND (N'Active' = [Extent2].[Status])
    )) OR ( NOT EXISTS (SELECT 
        1 AS [C1]
        FROM [dbo].[Pets] AS [Extent3]
        WHERE [Extent1].[OwnerID] = [Extent3].[OwnerID]
    ))) AND ( NOT EXISTS (SELECT 
        1 AS [C1]
        FROM [dbo].[Pets] AS [Extent4]
        WHERE ([Extent1].[OwnerID] = [Extent4].[OwnerID]) AND (N'Inactive' = [Extent4].[Status])
    ))

你想测试它的性能,可能有更好的方法,但它会给你想要的结果.它确实假设你有外键/导航属性.

Answer 2

select DISTINCT 
     o.Id 
FROM Owner o
LEFT JOIN Pet p ON o.OwnerID= p.OwnerID
where p.Status='Active' OR p.OwnerID IS NULL