Re: SQL Date Challenge

On Mon, 04 Jun 2001 10:31:52 -0700
"Josh Berkus" <josh(at)agliodbs(dot)com> wrote:
> Can anyone come up with a purely declarative (i.e. SQL) way to SELECT
> all of the Wednesdays within a given time period? Or is there, perhaps,
> some trick of the PGSQL date parser I could use?
>
> I can think of a number of ways to do this procedurally, but that's
> very awkward for what I need to use the information (to select all
> wednesdays within the last two months for which each staff emember has
> not turned in a timecard). I'm considering using a regularly updated
> reference table, but it seems like there *must* be a more elegant
> solution.
>
> Basically, what I want is:
>
> SELECT ALL Wednesdays BETWEEN 5/1/01 AND 6/1/01;
>
> Which results in:
>
> Wednesdays
> -----------
> 5/2/01
> 5/9/01
> 5/16/01
> 5/23/01
> 5/30/01

Try (under postgres 7.1)
select footable.somefield from footable where extract(dow from footable.my_date_field) = 3;

from the docs:

EXTRACT (field FROM source)

The extract function retrieves sub-fields from date/time values, such as year or hour. source is a value expression that
evaluates to type timestamp or interval. (Expressions of type date or time will be cast to timestamp and can therefore be used
as well.) field is an identifier (not a string!) that selects what field to extract from the source value. The extract function
returns values of type double precision. The following are valid values:
....
The day of the week (0 - 6; Sunday is 0) (for timestamp values only)

SELECT EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 5

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