SQL query joining tables from different fdb databases

As Mark notes you cannot join them directly. But you can still use a DSQL statement to get what you want.

Use execute block and execute statement together. Here's a sample.

execute block
returning (
   staffId integer,
   staffName varchar(100),
   remarks varchar(100)
   staffStatus varchar(10))
as
begin
   for SELECT staffId, staffName, remarks 
   FROM timeAtt 
   WHERE AB = 'X'
   into :staffId, :staffName, :remarks do begin

      execute statement 'select staffStatus from company where staffId = ' || staffId
      on external "your:connection:\string\and\db.fdb" as user FOO password BAR
      into :staffStatus;

      suspend;
   end
end

You can't. In Firebird you can only join tables in the same database file. Firebird 2.5 expanded EXECUTE STATEMENT to also execute a statement on an external datasource, but having a single query reference tables in different databases is not possible.

You have the following options:

1. Create a temporary table, copy the data you need into that temporary table and then join to the temporary table,
2. Merge the database into one.