procedure SetFilter(dbf :integer; s :string);

SetFilter sets a filter. The filter expression could be simple or complex.
The function does not change an order of records in a file. When a table is filtered then you cannot use some functions like DeleteRecord, InsertRecord.
You can use the following operations: =, <>, >, <, ==, ~.
The following math operators: +, -, *, /
& - means logical AND. | - means logical OR.

var
  dbf :integer;
begin
  dbf := OpenBase( 'filename.dbf' );
  if dbf <> 0 then
  begin
    SetFilter(dbf, 'NAME=Andy');
    showmessage( RecCount(dbf) );
    SetFilter(dbf, 'NAME=Andy&BIRTH>1980');
    showmessage( RecCount(dbf) );
    CloseBase(dbf);
  end;
end.