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Database variable in SQL Server 2008?

Posted on 2010-11-25
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Last Modified: 2012-05-10
In SQL Server 2008 (Express), I have a stored procedure that includes something like this

    SELECT * FROM DatabaseA.dbo.Table

I want to be able to do this instead

    SELECT * FROM @MyDB.dbo.Table

where @MyDB is a parameter or a variable whose value is DatabaseA

I know I can use Dynamic SQL to construct the select statement as a string and then execute the string. But that strikes me as a kludge and in any event is pretty awkward when I try to take a large procedure and generalize it to operate on a database that's selected at run time.

So, any way to do that? I know SQL Server 2008 has the notion of a table variable but I haven't seen anything that corresponds to a database variable. Any ideas?
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Question by:BlearyEye
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11 Comments
 
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Assisted Solution

by:Guy Hengel [angelIII / a3]
Guy Hengel [angelIII / a3] earned 100 total points
ID: 34213806
you cannot use a variable for a database name...
and yes, you are limited to use a sql string to execute it.
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by:BrandonGalderisi
ID: 34213848
As angel has pointed out, you cann't do that.  Dynamic SQL is the way.  The only alternative, which is still dynamic SQL so it's not really, but more a different way of doing it.


declare @MyDB sysname, @SQL nvarchar(max), @sp_executesql sysname

set @MyDB = 'SOMEDB'
set @SQL = 'select f1,f2,f3 from dbo.TableA'
set @sp_executesql = quotename(@MyDB) + '.dbo.sp_executesql'

exec @sp_ExecuteSQL @SQL
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Author Comment

by:BlearyEye
ID: 34219017
BrandonGalderisi: your code works, but I haven't been able to figure out why. Articles I've seen on sp_executesql don't use that syntax. Can you give me a pointer to an explanation?
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by:BrandonGalderisi
ID: 34231628
I am assigning @MyDB + '.dbo.sp_executesql' as the procedure to execute.  Perhaps my variable name has made it more confusing.


Maybe:

declare @MyDB sysname, @SQL nvarchar(max), @DYNAMICPROCEDURENAME sysname

set @MyDB = 'SOMEDB'
set @SQL = 'select f1,f2,f3 from dbo.TableA'
set @DYNAMICPROCEDURENAME= quotename(@MyDB) + '.dbo.sp_executesql'

exec @DYNAMICPROCEDURENAME @SQL
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Author Comment

by:BlearyEye
ID: 34235616
That wasn't really my issue. The exec, after unbundling the variables, is
   exec 'SOMEDB'.dbo.sp_executesql  'select f1, f2, f3 from dbo.TableA'

I don't understand the syntax and don't see how this becomes in effect
   select f1, f2, f3 from SOMEDB.dbo.TableA

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Expert Comment

by:BrandonGalderisi
ID: 34235734
Becaue it excutes the "select f1,f2,f3 from dbo.TableA" in SOMEDB because that's how the sp_executesql command was built.
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by:BlearyEye
ID: 34240110
I had looked for doc on sp_executesql (including specifically MSDN) and hadn't found anything that explicitly describes that usage. However, inferring from examples I've seen, it seems to be similar to the USE statement.

This would handle the original question I asked. However, I also need to paramaterize for two databases, with something like the following that copies the data from a table in DbB into the corresponding table in DbA.

   INSERT INTO DbA.dbo.CalibrationData SELECT * FROM DbB.dbo.CalibrationData

Should I use explicit parameters with sp_executesql?
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Expert Comment

by:BrandonGalderisi
ID: 34240358
There is no way to do that without putting one DB name into the @SQL.

This is not some magic syntax specific to sp_executesql.  You can do it with any procedure.


Ex.

declare @FooProc sysname
set @FooProc = 'up_DoFoo'
exec @FooProc
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Expert Comment

by:BrandonGalderisi
ID: 34240372
This is one of those situations where the correct answer is "you can't do that".  And in this case you must use Dynamic SQL.  Even using a variable for the @sp_executesql or @DYNAMICPROCEDURENAME is dynamic SQL.
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Author Comment

by:BlearyEye
ID: 34259212
So, I take the answer to be:

- Dynamic SQL is required
- Parameters for substituting database names are required if more than one database is involved.

If that's right, I'll close this question.
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Accepted Solution

by:
BrandonGalderisi earned 400 total points
ID: 34259478
Dynamic SQL is required:
Absolutely.  Even what I am doing is dynamic SQL.

Parameters for substituting database names are required if more than one database is involved:
If you want to reference another database, that name must be referenced in the executed SQL.  That reference may not be in the form of a variable, rather it must be part of the executed string.  Building the string by using the value of a variable in the place of a DB name does that.

So, yes.
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