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我使用的是 SQL Server 12.2.9 版（我认为是 SQL Server 2014）？

在 SQL 查询中，是否可以有一个或多个要从中选择数据的表名，作为将在查询执行期间评估的表达式？例如在下面的伪代码中？

SELECT * 
FROM MainTable AS mainTable, 
(
  /* Expression here that returns 
     a string (or what type do we return) 
     denoting the other table name 
  */ 
) AS AliasFoo
WHERE AliasFoo.Id = mainTable.ExternalId;

您能否提供此类查询的样本？具体来说，我们从应该返回对表/表名的引用的表达式返回什么数据类型？

问题的进一步发展

为了使这个例子更具体一点，以邀请适当的帮助，这里是一个人为的例子。

假设我有以下表格：

ActivityType
---------
Id ( int primary key, identity )
ActivityName (possible values are 'Jogging', 'Biking', and more)

ActivityLog
--------
Id ( int, primary key, identity) 
DateTime
ActivityTypeId
ActivityDetailId (a primary key of one of the following activity detail tables)

ACTIVITY DETAIL TABLES

Jogging
--------
Id ( int, primary key, identity) 
WhoWasJogging
ForHowLong
WhatShoesWereTheyWearing

Biking
--------
Id ( int, primary key, identity) 
WhoWasBiking
WhatBikeWasThat
WhatBrand
Color
Speed
ForHowLong

鉴于上述表格，我可以有这样的查询吗？

SELECT aLog.DateTime, aType.ActivityName, activityDetail.*
FROM ActivityLog AS aLog, ActivityType AS aType, 
(
  /*
  if ActivityType.ActivityName == 'Jogging' then the 'Jogging' table, 
  else if ActivityType.ActivityName == 'Biking' then the 'Biking' table
  */
) AS activityDetail
WHERE aLog.ActivityTypeId = aType.Id
AND activityDetail.Id = aLog.ActivityDetailId;
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1 回答 1

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好的，这是否是最佳答案取决于您在现实世界中有多少不同的表。因此，对于少量表left joining是一种可能的解决方案，如下所示。您可以看到这增加了选择列的复杂性，但这可能会满足您的需求。

select aLog.[DateTime]
  , aType.ActivityName
  , case when aType.ActivityName = 'Jogging' then J.WhoWasJogging else B.WhoWasBiking end WhoWas
  -- And so on
from ActivityLog as aLog
inner join ActivityType as aType on aType.Id = aLog.ActivityTypeId
left join Jogging as J on aType.ActivityName = 'Jogging' and aLog.ActivityDetailId = J.Id
left join Biking as B on aType.ActivityName = 'Biking' and aLog.ActivityDetailId = B.Id

它还取决于您是否要一次查询多个活动类型。

如果首选动态 SQL，那么以下应该可以工作：

declare @Sql nvarchar(max), @Activity varchar(128) = 'Biking';

set @Sql = 'select aLog.[DateTime]
  , aType.ActivityName
  , A.*
from ActivityLog as aLog
inner join ActivityType as aType on aType.Id = aLog.ActivityTypeId
inner join ' + @Activity + ' as A on and aLog.ActivityDetailId = A.Id
where aType.ActivityName = ''' + @Activity + '''';

exec (@sql);
于 2019-02-21T03:13:42.523 回答