Question

大家都在下面的查询中遇到问题。我有三个字段，其中Customer_id是客户的ID，GUID是客户的全局标识符，并且有DB_Parentid字段，其中包含客户的父客户的价值（如另一家公司的母公司）。

这里尝试使用下面的子查询填充'Customer Guid字段'中的GUID字段。以下查询可以从49 k记录中运行255行。相同的查询适用于另一个客户表。

select 

Customer_id as'SourceSystemID',

GUID as 'CustomerGUID',

(select a1.Customer_id from Customer a left join Customer a1 on a.DB_PARENT_ID=a1.Customer_id where a.Customer_id=Customer.Customer_id) AS 'Customer ParentGUID'

FROM dbo.Customer

Answer 1

为什么要在子查询中进行连接？我认为你的意图应该只是将它与外部查询相关联：

select Customer_id as SourceSystemID,
       GUID as CustomerGUID,
       (select parent.Customer_id
        from Customer parent 
        where c.DB_PARENT_ID = parent.Customer_id
       ) AS "Customer ParentGUID"
FROM dbo.Customer c;

我也认为最初的表述可能有倒退的逻辑，让孩子而不是父母回归。

最后，您通常会使用left outer join：

执行此操作

select Customer_id as SourceSystemID,
       GUID as CustomerGUID,
       parent.Customer_id AS "Customer ParentGUID"
FROM Customer c left outer join
     Customer parent
     on c.DB_PARENT_ID = parent.Customer_id

Answer 2

select a.Customer_id as'SourceSystemID',
a.GUID as 'CustomerGUID',
a1.Customer_id  AS 'Customer ParentGUID'
FROM dbo.Customer a
left join Customer a1 on a.DB_PARENT_ID=a1.Customer_id

子查询在以下查询中返回多个值

2 个答案: