Re: list of all months

On Mon, Mar 8, 2010 at 13:25, query <search2006(at)rediffmail(dot)com> wrote:
>
> Hi,
>
> I want to display data for all days in a month even if no data exists for that month. Some of the days in a month might not have any data at all. With normal query, we can display days only if data exists.But I want to display rows for all days in a month with blank data for non-existing day in database.
>
> How can this be achieved ?

Say, you have a table like:

CREATE TABLE some_data (
date date NOT NULL,
some_value int
);

Now, You would like to print values
from March 2010, even if there is no
entry for some days in such a table.

We need to have a list of all the days
in March. We can do it with a query:

SELECT date '2010-03-01' + n AS date
FROM generate_series(0, date '2010-04-01' - date'2010-03-01' - 1) AS x(n);

Sweet, we have dates, we just need a LEFT JOIN now:

SELECT date,
coalesce(value, 0) AS value
FROM some_data
RIGHT JOIN (
SELECT date '2010-03-01' + n AS date
FROM generate_series(0, date '2010-04-01' - date'2010-03-01' - 1) AS x(n)
) AS dates USING (date);

If you are running fairy recent PostgreSQL
it could be written even nicer:

WITH dates AS (
SELECT date '2010-03-01' + n AS date
FROM generate_series(0, date '2010-04-01' - date'2010-03-01' - 1) AS x(n)
)
SELECT date,
coalesce(value, 0) AS value
FROM dates
LEFT JOIN some_data USING (date);

Two remarks:
- it is fairy easy to create generate_series(date, date) function.
Give it a try - its fun! :)
- coalesce function will provide 0 in places where there is no
data row, or value is NULL.

Best regards,
Dawid