Re: BUG #1993: Adding/subtracting negative time

This is probably why the ANSI SQL standard specifies that
the way to get an INTERVAL from two timestamps is to subtract
them within parentheses follow that with interval_qualifier.  Under
the standard you are required to SAY whether you want the
interval in days or hours, etc.

*interval_value_expression
    :   interval_term
    |   interval_value_expression_1 PLUS_SIGN interval_term_1
    |   interval_value_expression_1 MINUS_SIGN interval_term_1
    |   LEFT_PAREN datetime_value_expression MINUS_SIGN datetime_term
RIGHT_PAREN interval_qualifier
    ;

*interval_qualifier
    :   start_field TK_to end_field
    |   single_datetime_field
    ;

*single_datetime_field
    :   non_second_datetime_field [ LEFT_PAREN
interval_leading_field_precision RIGHT_PAREN ]
    |   TK_second [ LEFT_PAREN interval_leading_field_precision [ COMMA
interval_fractional_seconds_precision ] RIGHT_PAREN ]
    ;

*non_second_datetime_field
    :   TK_year
    |   TK_month
    |   TK_day
    |   TK_hour
    |   TK_minute
    ;

*interval_leading_field_precision
    :   UNSIGNED_INTEGER
    ;

*interval_fractional_seconds_precision
    :   UNSIGNED_INTEGER
    ;

>>> Bruce Momjian <pgman@candle.pha.pa.us>  >>>

When you do a subtraction, it isn't clear if you are
interested in "days" or "hours"