Using variables in Transact-sql exists subquery - sql

Question

this seems like it should be extraordinarily simple, so I apologize in advance if this information is easily accessible on the transact-sql documentation pages. I searched myself, but couldn't seem to find anything.
I'm trying to modify a transact-sql statement that currently runs on our Windows server 2000 box. I want to check if a table in another database exists, and then do a bunch of stuff. The database name is given as a string argument, '#dbName'
CREATE PROCEDURE CopyTables
#dbName char(4)
AS
IF EXISTS (SELECT * FROM #dbName.INFORMATION_SCHEMA.TABLES WHERE
TABLE_NAME = N'MainTable')
BEGIN
--Do Stuff
In it's current state, it doesn't like using the bare #dbName variable within the select statement. Is there special syntax for doing this?
Thanks in advance.

Answer 1

The below code should do what you want. As was mentioned previously, the account running the query would need the privilege to query the INFORMATION_SCHEMAs in the target database.
To future-proof your stored procedure, I'd also suggest increasing the length of the database name parameter and declaring it as an nchar or nvarchar in stead of char.
CREATE PROCEDURE CopyTables
#dbName char(4)
AS
DECLARE
#SQLStr nvarchar (max),
#Params nvarchar (max),
#Count tinyint;
SET
#Count = 0;
SET #SQLStr = N'SELECT #qCount = 1 FROM [' + #dbName + N'].INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = N''MainTable''';
SET #Params = N'#qdbName char (4), #qCount tinyint OUTPUT';
EXECUTE sp_executesql #SQLStr, #Params, #qdbName = #dbName, #qCount = #Count OUTPUT;
IF #Count = 1
BEGIN
--Do Stuff
END; -- if
GO

Answer 2

Try doing the following:
DECLARE #dbName NVARCHAR(MAX) = 'master', #TableName NVARCHAR(MAX) = N'spt_monitor';
DECLARE #sql NVARCHAR(MAX) = N'SELECT * FROM [' + #dbName + N'].INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = ''' + REPLACE(#TableName,N'''',N'''''') + N'''';
SET NOCOUNT OFF;
EXEC(#sql);
IF ##ROWCOUNT > 0 BEGIN;
-- DO STUFF
SELECT NULL;
END;
There are a few shortcomings to this solution:
1) It requires that the user executing the statement has SELECT access to the other database's INFORMATION_SCHEMA.TABLES
2) It has the side-effect of actually selecting the rows, so if you're using a reader to access the results, you'll have to call reader.NextResult() or await reader.NextResultAsync() because it actually outputs the results of the SELECT statement, rather than doing it in an IF EXISTS context.

Answer 3

By merging the two solutions, we get this:
DECLARE #dbName NVARCHAR(MAX) = 'master', #TableName NVARCHAR(MAX) = N'spt_monitor';
DECLARE #sql NVARCHAR(MAX) = N'SELECT #count = COUNT(*) FROM [' + #dbName + N'].INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = ''' + REPLACE(#TableName,N'''',N'''''') + N'''';
DECLARE #Count INT;
EXECUTE sp_executesql #sql, N'#Count INT OUTPUT', #Count OUTPUT;
IF #Count > 0 BEGIN;
-- Do stuff
SELECT 'the table exists';
END ELSE BEGIN;
-- Do stuff
SELECT 'the table does not exist';
END;
This solution requires that the user executing the statement has SELECT access to the other database's INFORMATION_SCHEMA.TABLES, but it does not have the side-effect of selecting rows, like my previous solution.