连接两个表后选择列答案

【问题标题】：Selecting Columns after joining two tables连接两个表后选择列
【发布时间】：2019-04-09 03:40:44
【问题描述】：

我在加入两个表后尝试选择“contractkey”。我现在只能运行“select *”，然后返回所有列。下面是查询。只要我不尝试选择列，它就会完美运行。如何选择特定列？我需要在 INNER JOIN 之后标记表格吗？

Select * 
FROM 
(SELECT
    attachmentdata.contractkey,
    requirementinfo.reqcode,
    requirementinfo.parentidkey as 'ReqInfoParentID',
    max(attachment.datecreated) as datecreated,
    applicationinfo.hocompletiondate

FROM  location1.attachment

LEFT JOIN location1.attachmentdata
    on  attachment.contractkey = attachmentdata.contractkey
    and attachment.id = attachmentdata.parentidkey --Links Attachment Type

LEFT JOIN location1.requirementinfo
    on  attachment.contractkey = requirementinfo.contractkey
    and attachment.parentidkey = requirementinfo.id --Links Attachment Type

INNER JOIN location1.applicationinfo
    on  attachment.contractkey = applicationinfo.contractkey
    and requirementinfo.parentidkey = applicationinfo.parentidkey --Links Policy Type

where
    (requirementinfo.reqcode = 2) 

group by attachmentdata.contractkey,requirementinfo.reqcode, 
requirementinfo.parentidkey, applicationinfo.hocompletiondate) as table1

INNER JOIN

(SELECT
    attachmentdata.contractkey,
    requirementinfo.reqcode,
    requirementinfo.parentidkey as 'ReqInfoParentID',
    attachment.datecreated,
    applicationinfo.hocompletiondate,
    attachmentdata.pcdata

FROM  location1.attachment

LEFT JOIN location1.attachmentdata
    on  attachment.contractkey = attachmentdata.contractkey
    and attachment.id = attachmentdata.parentidkey --Links Attachment Type

LEFT JOIN location1.requirementinfo
    on  attachment.contractkey = requirementinfo.contractkey
    and attachment.parentidkey = requirementinfo.id --Links Attachment Type

INNER JOIN location1.applicationinfo
    on  attachment.contractkey = applicationinfo.contractkey
    and requirementinfo.parentidkey = applicationinfo.parentidkey)table2

on table1.contractkey = table2.contractkey
and table1.reqcode = table2.reqcode
and table1.ReqInfoParentID = table2.ReqInfoParentID
and table1.datecreated = table2.datecreated
and table1.hocompletiondate = table2.hocompletiondate

【问题讨论】：

请编辑您的问题并告诉我们如何失败。也许您在两个表之间有重复的列名。然后你需要指定你想要列的表：select table1.foo from (...)
是的。就是这样。我们复制的列！

标签： sql

【解决方案1】：

两个表中都有contractkey 列，因此系统不知道您想要哪些列。所以你必须把表名放在它前面。

SELECT table1.contractkey, ...

【讨论】：

如何在表名前面添加？
您使用table1 和table2 为您的两个查询设置别名。这就是您用来区分table1.contractkey 和table2.contractkey 的方法。

【解决方案2】：

将以下内容添加到最外层内表选择的末尾：

SELECT contractkey FROM
(SELECT
    attachmentdata.contractkey,

          etc...

) AS t(contractkey,reqcode,ReqInfoParentID,datecreated,hocompletiondate)

【讨论】：

【解决方案3】：

不需要“给表格加标签”（别名）。用例确保它不会为不在 AttachmentData 表中的记录设置 null，如下所示：

Select *  FROM 
(SELECT case 
   WHEN attachmentdata.contractkey IS NOT NULL THEN attachmentdata.contractkey 
   else attachment.contractkey END AS contractkey,

    requirementinfo.reqcode,
    requirementinfo.parentidkey as 'ReqInfoParentID',
    max(attachment.datecreated) as datecreated,
    applicationinfo.hocompletiondat
    ...

【讨论】：

你知道OP的问题吗？