Dynamically build a UNION ALL

Hi,

In a SQL Server database there are multiple tables. I want to search for a certain string-value accross these tables using a Stored Procedure. The result-set from each table will be exactly the same and is currently combined using "UNION ALL".
Depending on the authorization of a user (in my Delphi app) I only want to select records from certain tables. So I was thinking about using a few parameters to pass that information.
If a user is not authorized for any of the tables the resultset will be empty.

CREATE PROCEDURE MyProcedure (
      @SearchString  nvarchar(255),
      @AllowTable1  bit,
      @AllowTable2  bit,
      @AllowTable3  bit
      )
AS 
BEGIN 
  -- Based on parameter @AllowTable1 I want to add values from Table1 to the selection (or not).
  SELECT MyID1 AS ID, MyNvarcharField1 AS FoundString, 'Table1' AS Table 
  FROM Table1
  WHERE MyNvarcharField1 LIKE '%' + @SearchString + '%'
  UNION ALL
  -- Based on parameter @AllowTable2 I want to add values from Table2 to the selection (or not).
  SELECT MyID2 AS ID, MyNvarcharField2 AS FoundString, 'Table2' AS Table 
  FROM Table2
  WHERE MyNvarcharField2 LIKE '%' + @SearchString + '%'
  UNION ALL
  -- Based on parameter @AllowTable3 I want to add values from Table3 to the selection (or not).
  SELECT MyID3 AS ID, MyNvarcharField3 AS FoundString, 'Table3' AS Table 
  FROM Table3
  WHERE MyNvarcharField3 LIKE '%' + @SearchString + '%'
  Ect.
END

Select allOpen in new window

What would be the right way to accomplish this?
Any different approach is perfectly fine for me too.