Aggregates for today and the previous day depending on data - sql

Question

Having trouble putting together a query to pull the aggregate values of a give timestamp and the timestamp before it. Given the following schema:
name TEXT,
ts TIMESTAMP,
X NUMERIC,
Y NUMERIC
where there are gaps in the ts column due to gaps in data, I'm trying to construct a query to produce
name,
date_trunc('day' q1.ts),
avg(q1.X),
sum(q2.Y),
date_trunc('day', q2.ts),
avg(q2.X),
sum(q2.Y)
The first half is straightforward:
SELECT q1.name, date_trunc('day', q1.ts), avg(q1.X), sum(q1.Y)
FROM data as q1
GROUP BY 1, 2
ORDER BY 1, 2;
But not sure how to generate the relation to find the "day" before for each row. I'm trying to work an inner join like this:
SELECT q1.name, q1.day, q1.avg, q1.sum, q2.day, q2.avg, q2.sum
FROM (
SELECT name, date_trunc('day', ts) AS day, avg(X) AS avg, sum(Y) as sum
FROM data
GROUP BY 1,2
ORDER BY 1,2
) q1 INNER JOIN (
SELECT name, date_trunc('day', ts) AS day, avg(X) AS avg, sum(Y) as sum
FROM data
GROUP BY 1,2
ORDER BY 1,2
) q2 ON (
q1.name = q2.name
AND q2.day = q1.day - interval '1 day'
);
The problem with this is, it doesn't cover the cases when the next "day" is more than 1 day before the current day.

Answer 1

The special difficulty here is that you need to number days after aggregating rows. You can do this in a single query level with the window function row_number(), since window functions are applied after aggregation by GROUP BY.
Also, use a CTE to avoid executing the same subquery multiple times:
WITH q AS (
SELECT name, ts::date AS day
,avg(x) AS avg_x, sum(y) AS sum_y
,row_number() OVER (PARTITION BY name ORDER BY ts::date) AS rn
FROM data
GROUP BY 1,2
)
SELECT q1.name, q1.day, q1.avg_x, q1.sum_y
,q2.day AS day2, q2.avg_x AS avg_x2, q2.sum_y AS sum_y2
FROM q q1
LEFT JOIN q q2 ON q1.name = q2.name
AND q1.rn = q2.rn + 1
ORDER BY 1,2;
Using the simpler cast to date (ts::date) instead of date_trunc('day', ts) to get "days".
LEFT [OUTER] JOIN (as opposed to [INNER] JOIN) is instrumental to preserve the corner case of the first row, where there is no previous day.
And ORDER BY should be applied to the outer query.

Answer 2

The question isn't crystal clear, but it sounds like you're actually trying to fill gaps while keeping track of leading/lagging rows.
To fill the gaps, look into generate_series() and left join it with your table:
select d
from generate_series(timestamp '2013-12-01', timestamp '2013-12-31', interval '1 day') d;
http://www.postgresql.org/docs/current/static/functions-srf.html
For previous and next row values, look into lead() and lag() window functions:
select date_trunc('day', ts) as curr_row_day,
lag(date_trunc('day', ts)) over w as prev_row_day
from data
window w as (order by ts)
http://www.postgresql.org/docs/current/static/tutorial-window.html