Question

i would like to find/mark every 4th day in a continuous date stream inserted into my table for each user in a given date range

CREATE TABLE mytable (
  id INTEGER,
  myuser INTEGER,            
  day DATE NOT NULL,
  PRIMARY KEY  (id)      
);

the problem is, that only 3 continuous days are valid per user, after that, there has to be a one day “break”

 id  | myuser |    day     |
-----+--------+------------+
  0  |    200 | 2012-01-12 | }
  1  |    200 | 2012-01-13 | }--> 3 continuous days
  2  |    200 | 2012-01-14 | }
  3  |    200 | 2012-01-15 | <-- not ok, user 200 should get warned and delete this
  4  |    200 | 2012-01-16 | }
  5  |    200 | 2012-01-17 | }--> 3 continuous days
  6  |    200 | 2012-01-18 | }
  7  |    200 | 2012-01-19 | <-- not ok, user 200 should get warned and delete this
  8  |    201 | 2012-01-12 | }
  9  |    201 | 2012-01-13 | }--> 3 continuous days
  10 |    201 | 2012-01-14 | }
  11 |    201 | 2012-01-16 | <-- ok, there is a one day gap here
  12 |    201 | 2012-01-17 | 

the main goal is to look at a given date range (usually a month) and identify days, which are not allowed. Also i have to take care that the overlapping dates are handled correctly, for example, if i look on a date range from 2012-02-01 to 2012-02-29, 2012-02-01 could be a “break” day if 2012-01-29 to 2012-01-31 is present in that table for the same user.

Answer 1

I don’t have access to PostgreSQL, but hopefully this works…

WITH
  grouped_data AS
(
  SELECT
    ROW_NUMBER() OVER (PARTITION BY myuser ORDER BY day) - (day - start_date) AS user_group_id,
    myuser,
    day
  FROM
    myTable
  WHERE
        day >= start_date - 3
    AND day <= end_date
)
,
  sequenced_data AS
(
  SELECT
    ROW_NUMBER() OVER (PARTITION BY myuser, user_group_id ORDER BY day) AS sequence_id,
    myuser,
    day
  FROM
    grouped_data
)
SELECT
  myuser,
  day,
  CASE WHEN sequence_id % 4 = 0 THEN 1 ELSE 0 END as should_be_a_break_day
FROM
  sequenced_data
WHERE
  day >= start_date

Sorry I didn’t explain the workings, I had to jump into a meeting 🙂

Example with start_date = ‘2012-01-14’…

 id | myuser |    day     | ROW_NUMBER() | day - start_date | user_group_id
----+--------+------------+--------------+------------------+---------------
  0 |    200 | 2012-01-12 |     1        |        -2        |  1 - -2 = 3
  1 |    200 | 2012-01-13 |     2        |        -1        |  2 - -1 = 3
  2 |    200 | 2012-01-14 |     3        |         0        |  3 -  0 = 3
  3 |    200 | 2012-01-15 |     4        |         1        |  4 -  1 = 3
  4 |    200 | 2012-01-16 |     5        |         2        |  5 -  2 = 3
----+--------+------------+--------------+------------------+---------------
  5 |    201 | 2012-01-12 |     1        |        -2        |  1 - -2 = 3
  6 |    201 | 2012-01-13 |     2        |        -1        |  2 - -1 = 3
  7 |    201 | 2012-01-14 |     3        |         0        |  3 - -1 = 3
  8 |    201 | 2012-01-16 |     4        |         2        |  4 -  2 = 2

Any sequential dates will have the same user_group_id. Each ‘gap’ in the days makes that user_group_id decrease by 1 (see row 8, if the record was for the 17th, a 2 day gap, the id would have been 1).

Once you have a group_id, row_number() can be easily used to say which day in the sequence it is. A max of 3 day is the same as “Every 4th day should be a gap”, and “x % 4 = 0” identifies every 4th day.