查询要从中选择数据的 table 名称是表达式？

Question

我正在使用 SQL 服务器版本 12.2.9（即 SQL 服务器 2014，我认为）？

在 SQL 查询中，是否可以使用一个或多个要从中选择数据的 table 名称作为将在查询执行期间计算的表达式？例如如以下伪代码？

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 return 引用 table / table 名称的表达式中 return 是什么数据类型？

问题的进一步发展

为了使示例更具体一些以邀请适当的帮助，这里是一个人为的示例。

假设我有以下 tables:

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 tables，我可以这样查询吗？

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;

Answer 1

好的，这是否是最佳答案取决于您在现实世界中有多少个不同的表。因此，对于少量表，left joining 是一种可能的解决方案，如下所示。您可以在 select 列中看到这增加了复杂性，但这可能会给您想要的。

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

这还取决于您是否希望一次查询多个 activity 类型。

如果动态 SQL 是首选，那么以下应该有效：

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);

查询要从中选择数据的 table 名称是表达式？

Query to have the table name from which data is to be selected be an expression?

sql

sql-server

sql-server-2014