No idea. I don't know anything about Yapp (or yacc, for that matter). If you really think database_ident is likely to change, you could go with:

database_ident:
    ident
;

table_ident:
    ident
;

column_ident:
    ident
;

qualified_database_ident:
    database_ident
;

qualified_table_ident:
    database_ident '.' table_ident
  |                    table_ident
;

qualified_column_ident:
    database_ident '.' table_ident '.' column_ident
  |                    table_ident '.' column_ident
  |                                    column_ident
;
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