Question

Table structure – Data present for 5 min. slots –

data_point | point_date

12 | 00:00

14 | 00:05

23 | 00:10

10 | 00:15

43 | 00:25

10 | 00:40

When I run the query for say 30 mins. and if data is present I’ll get 6 rows (one row for each 5 min. stamp). Simple Query –

select data_point
from some_table
where point_date >= start_date
AND point_date < end_date
order by point_date

Now when I don’t have an entry for a particular time slot (e.g. time slot 00:20 is missing), I want the “data_point” to be returned as 0

The REPLACE, IF, IFNULL, ISNULL don’t work when there no rows returned.

I thought Union with a default value would work, but it failed too or maybe I didn’t use it correctly.

Is there a way to get this done via sql only ?

Note : Python 2.6 & mysql version 5.1

Answer 1

Yes, you can do that using SQL only. A solution would be to use a Stored Routine. The bellow Stored Procedure produces following output:

start   cnt
00:05:00   1
00:10:00   0
00:15:00   1
00:20:00   0
00:25:00   1
00:30:00   0
00:35:00   1
00:40:00   0
00:45:00   0
00:50:00   0
00:55:00   2

The table I used:

CREATE TABLE `timedata` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `c1` datetime DEFAULT NULL,
  `c2` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
)

Here the Stored Procedure (adjust for your environment):

DROP PROCEDURE IF EXISTS per5min;
DELIMITER //
CREATE PROCEDURE per5min ()
BEGIN
  DECLARE dtMin DATETIME;
  DECLARE dtMax DATETIME;
  DECLARE dtStart DATETIME;
  DECLARE dtStop DATETIME;
  DECLARE tmDiff TIME;
  DECLARE result INT UNSIGNED;
  SET @offset = 5 * 60;
  SELECT MIN(c1) into dtMin FROM timedata;
  SELECT MAX(c1) into dtMax FROM timedata;

  CREATE TEMPORARY TABLE tmp_per5min (
      start TIME,
      cnt INT UNSIGNED
  );

  SET dtStart = dtMin;
  REPEAT
    SELECT dtStart + INTERVAL @offset SECOND into dtStop;
    SELECT count(c2) into result FROM timedata WHERE c1 BETWEEN dtStart and dtStop;
    SELECT TIME(SUBTIME(dtStop,TIME(dtMin))) into tmDiff;
    INSERT INTO tmp_per5min (start,cnt) VALUES (tmDiff,result);
    SET dtStart = dtStop;
  UNTIL dtStop >= dtMax END REPEAT;

  SELECT * FROM tmp_per5min;
  DROP TABLE tmp_per5min;
END;
//
DELIMITER ;

CALL per5min();

If you save the above into a file called ‘per5minproc.sql’, you can load it like this:

shell> mysql -uroot test < per5minproc.sql

In Python using MySQLdb (I didn’t get this working in MySQL Connector/Python, me ashamed!):

import MySQLdb as m

if __name__ == '__main__':
    db = m.connect(user='root',db='test')
    c = db.cursor()
    c.callproc("per5min")
    print(c.fetchall())
    c.close()
    db.close()

The solution above works, but probably will need some tweaking, e.g. dtStart can be an argument to the SP.
And, it’s indeed all SQL!