Question

我正在使用SQL Server 12.2.9版（我认为是SQL Server 2014）？

在SQL查询中，是否可以具有一个或多个要从中选择数据的表名，使其成为在查询执行期间将被评估的表达式？例如像下面的伪代码一样？

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;

能否请您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中返回什么数据类型？

问题的进一步发展

为了使示例更具体一些，以吸引适当的帮助，这是一个人为的示例。

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

鉴于上面的表格，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

好吧，这是否是最佳答案，取决于您在现实世界中有多少张不同的表。因此，对于少量表left joining可能是一种解决方案，如下所示。您可以在选择列中看到这增加了复杂性，但这可能会给您您想要的。

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

这还取决于您是否要一次查询多个活动类型。

如果首选动态sql，则应该可以执行以下操作：

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)

在SQL查询中是否可以将要从中选择数据的表名作为表达式？

1 个答案: