Question

I have the following query to count all data every minute.

$sql= "SELECT COUNT(*) AS count, date_trunc('minute', date) AS momento
FROM p WHERE fk_id_b=$id_b GROUP BY date_trunc('minute', date) 
ORDER BY momento ASC";

What I need to do is get the sum of the count for each row with the count of the 2 past minutes.

For example with the result of the $sql query above
|-------date---------|----count----|
|2012-06-21 05:20:00 |      12     |
|2012-06-21 05:21:00 |      14     |
|2012-06-21 05:22:00 |      10     |
|2012-06-21 05:23:00 |      20     |
|2012-06-21 05:24:00 |      25     |
|2012-06-21 05:25:00 |      30     |
|2012-06-21 05:26:00 |      10     |  

I want this result:

|-------date---------|----count----|
|2012-06-21 05:20:00 |      12     |   
|2012-06-21 05:21:00 |      26     |     12+14
|2012-06-21 05:22:00 |      36     |     12+14+10
|2012-06-21 05:23:00 |      44     |     14+10+20
|2012-06-21 05:24:00 |      55     |     10+20+25
|2012-06-21 05:25:00 |      75     |     20+25+30
|2012-06-21 05:26:00 |      65     |     25+30+10

Answer 1

This is not so tricky with lag() window function (also on SQL Fiddle):

CREATE TABLE t ("date" timestamptz, "count" int4);
INSERT INTO t VALUES
('2012-06-21 05:20:00',12),
('2012-06-21 05:21:00',14),
('2012-06-21 05:22:00',10),
('2012-06-21 05:23:00',20),
('2012-06-21 05:24:00',25),
('2012-06-21 05:25:00',30),
('2012-06-21 05:26:00',10);

SELECT *,
    "count"
    + coalesce(lag("count", 1) OVER (ORDER BY "date"), 0)
    + coalesce(lag("count", 2) OVER (ORDER BY "date"), 0) AS "total"
  FROM t;

1. I’ve double-quoted date and count columns, as these are reserved words;
2. lag(field, distance) gives me the value of the field column distance rows away from the current one, thus first function gives previous row’s value and second call gives the value from the one before;
3. coalesce() is required to avoid NULL result from lag() function (for the first row in your query there’s no “previous” one, thus it’s NULL), otherwise the total will also be NULL.