Question

我试图在连接两个表后选择“ 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，它就可以完美运行。如何选择特定的列？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

Answer 1

两个表中都有contractkey列，因此系统不知道您要哪个列。因此，您必须将表名放在其前面。

SELECT table1.contractkey, ...

Answer 2

在您最外面的内部表选择的末尾添加以下内容：

SELECT contractkey FROM
(SELECT
    attachmentdata.contractkey,

          etc...

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

Answer 3

不需要“标记表格”（别名）。用例确保对于不在AttachmentData表中的记录，它不会为空，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
    ...

连接两个表后选择列

3 个答案: