Count value variation from 0 to 1 in mysql table - mysql

Question

I have a column with two columns. one is TIMESTAMP and the other DIGITAL_BIT.
The value digital bit can be either 0 or 1 and changes a few times during the day. Every minute of the day is stored in this table. I would need to read somehow how many times a day this value changed from 0 to 1.
Is it possible to make a query that returns the count of this changes? What I have in mind is something like this:
select * from mytable where digital_bit = 1 and digital_bit (of previous row) = 0 order by timestamp
Can this be done with a query or do i have to process all data in my program?
Thanks
SAMPLE
timestamp | digital_bit
100000 | 0
100001 | 0
100002 | 1
100003 | 1
100004 | 0
100005 | 1
100006 | 0
100007 | 0
100008 | 1
the above should return 3 because for 3 times the value digital passed from 0 to 1. i need to count how often the value digital CHANGES from 0 to 1.

Answer 1

Here you go. This will get you a count of how many times digital_bit switched from 0 to 1 (in your example, this will return 3).
SELECT COUNT(*)
FROM mytable curr
WHERE curr.digital_bit = 1
AND (
SELECT digital_bit
FROM mytable prev
WHERE prev.timestamp < curr.timestamp
ORDER BY timestamp DESC
LIMIT 1
) = 0
SQLFiddle link
(Original answer relied on the timestamps being sequential: e.g. no jumps from 100001 to 100003. Answer has now been updated not to have that restriction.)

Answer 2

IF you have a result once per minte, you can simple join the table with itself, and
use timestamp+1 as well as leftbit != rightbit as join condition.
http://sqlfiddle.com/#!8/791c0/6
ALL Changes:
SELECT
COUNT(*)
FROM
test a
INNER JOIN
test b
ON
a.digital_bit != b.digital_bit
AND b.timestamp = a.timestamp+1;
Changes from 0 to 1
SELECT
COUNT(*)
FROM
test a
INNER JOIN
test b
ON
a.digital_bit = 0 AND
a.digital_bit != b.digital_bit
AND b.timestamp = a.timestamp+1;
Changes from 1 to 0
SELECT
COUNT(*)
FROM
test a
INNER JOIN
test b
ON
a.digital_bit = 1 AND
a.digital_bit != b.digital_bit
AND b.timestamp = a.timestamp+1;

Answer 3

Adapted from: How do I query distinct values within multiple sub record sets
select count(*)
from (select t1.*,
(select digital_bit
from table t2
where t2.timestamp < t1.timestamp
order by timestamp desc LIMIT 1
) as prevvalue
from table t1
) t1
where prevvalue <> digital_bit and digital_bit = 1;

Answer 4

This isn't likely to be efficient with a lot of data, but you can get all the rows and calculate a sequence number for them, then do the same again but with the sequence number offset by 1. Then join the 2 lots together where those calculated sequence numbers match but the first one has a digital bit of 0 and the other a digital bit of 1:-
SELECT COUNT(*)
FROM
(
SELECT mytable.timestamp, mytable.digital_bit, #aCount1:=#aCount1+1 AS SeqCount
FROM mytable
CROSS JOIN (SELECT #aCount1:=1) sub1
ORDER BY timestamp
) a
INNER JOIN
(
SELECT mytable.timestamp, mytable.digital_bit, #aCount2:=#aCount2+1 AS SeqCount
FROM mytable
CROSS JOIN (SELECT #aCount2:=0) sub1
ORDER BY timestamp
) b
ON a.SeqCount = b.SeqCount
AND a.digital_bit = 0
AND b.digital_bit = 1
EDIT - alternative solution and I would be interested to see how this performs. It avoids the need for adding a sequence number and also avoids a correlated sub query:-
SELECT COUNT(*)
FROM
(
SELECT curr.timestamp, MAX(curr2.timestamp) AS MaxTimeStamp
FROM mytable curr
INNER JOIN mytable curr2
ON curr.timestamp > curr2.timestamp
AND curr.digital_bit = 1
GROUP BY curr.timestamp
) Sub1
INNER JOIN mytable curr
ON Sub1.MaxTimeStamp = curr.timestamp
AND curr.digital_bit = 0

Answer 5

As I understood you have one query every minute. So you have no problem with performance.
You can add flag:
timestamp | digital_bit | changed
100000 | 0 | 0
100001 | 0 | 0
100002 | 1 | 1
100003 | 1 | 0
100004 | 0 | 1
100005 | 1 | 1
100006 | 0 | 1
100007 | 0 | 0
100008 | 1 | 1
And make check before insert:
SELECT digital_bit
FROM table
ORDER BY timestamp DESC
LIMIT 1
and if digital_bit is different insert new row with flag.
And then you just can take COUNT of flags:
SELECT COUNT(*)
FROM table
WHERE DATE BETWEEN (start, end)
AND changed = 1
Hope will see in answers better solution.