MySQL Select Day Streak

Question

This question is similar to the one answered here: http://sqlfiddle.com/#!2/929846/1/0

Id    UserId  Timestamp
1     1       2012-12-01
2     2       2012-12-02
3     1       2012-12-02
4     2       2012-12-02
5     1       2012-12-03
6     3       2012-12-02
7     2       2012-12-03
8     3       2012-12-03

I'd like to retrieve the current streak for a user 1 . The expected result is 3 if today's date is 2012-12-03 .

The user will never have more than one entry in the same space of 24 hours.

If they have 4 records, their streak is 4. If they have 2 records, their streak is 2. If their last record is more than 24 hours ago, their streak is 0.

How can I calculate the streak using just a MySQL query?

Rephrase

A "streak" is defined as the number of consecutive days ( Timestamp ) having an entry for a given UserId .

Need help writing sql (or maybe a Stored Function) that, given a UserId and an ending Timestamp , returns the streak .

Answer 1

You can use the following query:

SELECT MAX(streak) AS streak
FROM (
  SELECT Id, UserId, `Timestamp`,
         DATEDIFF(NOW(), `Timestamp`),
         @streak := IF( DATEDIFF(NOW(), `Timestamp`) - @days_diff > 1, @streak, 
                       IF(@days_diff := DATEDIFF(NOW(), `Timestamp`), @streak+1, @streak+1))  AS streak                                        
  FROM mytable
  CROSS JOIN (SELECT @streak := 0, @days_diff := -1) AS vars
  WHERE UserId = 1 AND `Timestamp` <= NOW()
  ORDER BY `Timestamp` DESC) AS t

The query uses @streak variable to incrementally update the streak number. The variable keeps being incremented as long as there is no gap bigger than one day between two consecutive records.

Note that @days_diff is initialized to -1 so that if the last record, which is processed first , is more than 24 hours ago, the streak is set to 0 .

Demo here