如何正确连接可能返回多行的表

Question

我有一个表 Items，其中包含以下列：

项目ID ||管理员用户ID ||物品名称

我尝试使用 ItemAttributes 加入它：

ItemAttributeId ||项目编号 ||属性值

其中一个 ItemId 可以有多个 ItemAttributeId 值分配给它。

我创建了从两个表中检索项目值的过程：

 SELECT 
    i.ItemID,
    i.AdminUserID,
    i.ItemName,
    attr1.AttributeValue as IsForEmailRobotProcessingValue,
    attr2.AttributeValue as RobotScheduledHoursValue -- HERE IS THE PROBLEM
FROM [dbo].[Items] i WITH (NOLOCK)
    LEFT JOIN [dbo].ItemAttributes attr1 WITH(NOLOCK)
        ON attr1.ItemID = i.ItemID and attr1.ItemAttributeID = 1
    LEFT JOIN [dbo].ItemAttributes attr2 WITH(NOLOCK)
    ON attr2.ItemID = i.ItemID and attr2.ItemAttributeID = 2
WHERE ((attr1.AttributeValue IS NOT NULL AND attr1.AttributeValue = 'true') OR 
    @OnlyForEmailRobotProcessing = 0)
ORDER BY ItemName

我不喜欢这里的是我 LEFT JOIN 两次同一个表，并根据相同的 ItemId 为每个连接创建了不同的别名。有没有更好的方法来 LEFT JOIN 一次并为每个 ItemAttributeId 创建两个不同的别名？谢谢！

Answer 1

试试这个：

SELECT 
    i.ItemID,
    i.AdminUserID,
    i.ItemName,
    attr1.AttributeValue1 as IsForEmailRobotProcessingValue,
    attr1.AttributeValue2 as RobotScheduledHoursValue 
FROM [dbo].[Items] i WITH (NOLOCK)
    LEFT JOIN
            (select ItemId,  (select AttributeValue 
                              from  [dbo].ItemAttributes a
                              where a.ItemId = b.ItemId and a.ItemAttributeID = 1) as AttributeValue1,
                              (select AttributeValue 
                              from  [dbo].ItemAttributes c
                              where c.ItemId = b.ItemId and c.ItemAttributeID = 2) as AttributeValue2
             from    [dbo].ItemAttributes b  WITH(NOLOCK)
             where b.ItemId = i.ItemID  and b.ItemAttributeID in (1,2)

                ) attr1 on i.ItemId  = attr1.ItemId

WHERE ((attr1.AttributeValue IS NOT NULL AND attr1.AttributeValue = 'true') OR 
    @OnlyForEmailRobotProcessing = 0)
ORDER BY ItemName