Re: [SQL] Using dates - Mailing list pgsql-sql

> Perhaps if you explain exactly what you want returned from your query I
> will better understand your problem. What you have now describes shows a
> problem in understanding the semantics of date arithmetic.
> 
> 1) Comparing two dates: As far as I know, this worked ever since
>    6.1, which is the first version of Postgres I used. Just make
>    sure that both sides are indeed dates! It sounds to me as if
>    your trouble is that one of the side was mistaken for an
>    integer.
> 
>    ... WHERE d_start < '1999-04-14'::date
> 
>    should work well.
> 
> 2) Subtracting integer from date. The semantics of this operation
>    is usually that the integer is number of days.
> 
>     testing=> select d_start, d_start - 30 from test2;
>        d_start|  ?column?
>     ----------+----------
>     01-15-1969|12-16-1968
>     07-14-1999|06-14-1999
>     04-13-1998|03-14-1998
>     12-01-1999|11-01-1999
>     (4 rows)
> 
>     The result is of type date even if your original d_start was
>     of type datetime and not date.
> 
> 3) Subtracting a date from a date, as I said, gives you either a
>    timespan or an integer, depending on the type of the operands.
>    You must know how to format a correct date. Just writing
>    1999-4-7 without quotation marks, and preferably adding
>    ::date will probably not pass the parser, even. I'm not sure
>    the function date() worked in early versions.
> 
> 4) Another option for date subtraction is the function age().
> 
>     testing=> select d_start, d_end, age( d_end, d_start ) from test2;
>        d_start|     d_end|age
>     ----------+----------+----------------------------------
>     01-15-1969|08-01-1999|@ 30 years 6 mons 16 days 23 hours
>     07-14-1999|07-18-1999|@ 4 days
>     04-13-1998|03-12-1998|@ 1 mon 23 hours ago
>     12-01-1999|12-01-1999|@ 0
>     (4 rows)
> 
>     This function returns a timespan, for date or datetime operands.
>     The main difference between this and the timespan returned
>     with simple datetime subtraction is that it converts to years,
>     months and days, instead of just days and hours.
> 
> 5) Finally, remember that you can compare timespans. Thus, you can
>    ask for all tuples two weeks back or later, for example, using:
> 
>    testing=> SELECT * FROM test1 WHERE ( 'now' - t_end ) < '2 weeks';
>    t_start                     |t_end
>    ----------------------------+----------------------------
>    Wed Jan 15 00:00:00 1969 IST|Sun Aug 01 00:00:00 1999 IDT
>    Wed Dec 01 00:00:00 1999 IST|Wed Dec 01 00:00:00 1999 IST
>    (2 rows)
> 
>    Or, in a better way for utilizing indices:
> 
>    testing=> SELECT * FROM test1
>    testing-> WHERE t_end > ( 'now'::datetime - '2 weeks'::timespan );
>    t_start                     |t_end
>    ----------------------------+----------------------------
>    Wed Jan 15 00:00:00 1969 IST|Sun Aug 01 00:00:00 1999 IDT
>    Wed Dec 01 00:00:00 1999 IST|Wed Dec 01 00:00:00 1999 IST
>    (2 rows)
> 
>    You noticed that Dec 01 1999 is in the future? Then makse sure
>    to add alse t_end < 'now'.
> 
> HTH,
> Herouth

Guess this stuff is a long way from the dot notation I learned from
Paradox PAL.

Now busy reading in depth the Programmer Manual.

Can you suggest any further reading

Get the feeling that there are more than myself, reading this digest,
that would appreciate such a tip.

Many thanks for the help - It has definitely headed me in the right
direction.

Max Wood