CODEWITHSUNDEEP
mysqlsql
Chris Garson
Chris Garson

Reputation: 199

Checking for maximum length of consecutive days which satisfy specific condition

I have a MySQL table with the structure:

beverages_log(id, users_id, beverages_id, timestamp)

I'm trying to compute the maximum streak of consecutive days during which a user (with id 1) logs a beverage (with id 1) at least 5 times each day. I'm pretty sure that this can be done using views as follows:

CREATE or REPLACE VIEW daycounts AS
SELECT count(*) AS n, DATE(timestamp) AS d FROM beverages_log
WHERE users_id = '1' AND beverages_id = 1 GROUP BY d;

CREATE or REPLACE VIEW t AS SELECT * FROM daycounts WHERE n >= 5;

SELECT MAX(streak) AS current FROM ( SELECT DATEDIFF(MIN(c.d), a.d)+1 AS streak
FROM t AS a LEFT JOIN t AS b ON a.d = ADDDATE(b.d,1)
LEFT JOIN t AS c ON a.d <= c.d
LEFT JOIN t AS d ON c.d = ADDDATE(d.d,-1)
WHERE b.d IS NULL AND c.d IS NOT NULL AND d.d IS NULL GROUP BY a.d) allstreaks;

However, repeatedly creating views for different users every time I run this check seems pretty inefficient. Is there a way in MySQL to perform this computation in a single query, without creating views or repeatedly calling the same subqueries a bunch of times?

Upvotes: 5

Views: 800

Answers (3)

user1191247
user1191247

Reputation: 12973

This solution seems to perform quite well as long as there is a composite index on users_id and beverages_id -

SELECT *
FROM (
    SELECT t.*, IF(@prev + INTERVAL 1 DAY = t.d, @c := @c + 1, @c := 1) AS streak, @prev := t.d
    FROM (
        SELECT DATE(timestamp) AS d, COUNT(*) AS n
        FROM beverages_log
        WHERE users_id = 1
        AND beverages_id = 1
        GROUP BY DATE(timestamp)
        HAVING COUNT(*) >= 5
    ) AS t
    INNER JOIN (SELECT @prev := NULL, @c := 1) AS vars
) AS t
ORDER BY streak DESC LIMIT 1;

Upvotes: 6

user565869
user565869

Reputation:

That's a little tricky. I'd start with a view to summarize events by day:

CREATE VIEW BView AS
    SELECT UserID, BevID, CAST(EventDateTime AS DATE) AS EventDate, COUNT(*) AS NumEvents
    FROM beverages_log
    GROUP BY UserID, BevID, CAST(EventDateTime AS DATE)

I'd then use a Dates table (just a table with one row per day; very handy to have) to examine all possible date ranges and throw out any with a gap. This will probably be slow as hell, but it's a start:

SELECT
    UserID, BevID, MAX(StreakLength) AS StreakLength
FROM
    (
    SELECT
        B1.UserID, B1.BevID, B1.EventDate AS StreakStart, DATEDIFF(DD, StartDate.Date, EndDate.Date) AS StreakLength
    FROM
        BView AS B1
        INNER JOIN Dates AS StartDate ON B1.EventDate = StartDate.Date
        INNER JOIN Dates AS EndDate ON EndDate.Date > StartDate.Date
    WHERE
            B1.NumEvents >= 5
        -- Exclude this potential streak if there's a day with no activity
        AND NOT EXISTS (SELECT * FROM Dates AS MissedDay WHERE MissedDay.Date > StartDate.Date AND MissedDay.Date <= EndDate.Date AND NOT EXISTS (SELECT * FROM BView AS B2 WHERE B1.UserID = B2.UserID AND B1.BevID = B2.BevID AND MissedDay.Date = B2.EventDate))
        -- Exclude this potential streak if there's a day with less than five events
        AND NOT EXISTS (SELECT * FROM BView AS B2 WHERE B1.UserID = B2.UserID AND B1.BevID = B2.BevID AND B2.EventDate > StartDate.Date AND B2.EventDate <= EndDate.Date AND B2.NumEvents < 5)
    ) AS X
GROUP BY
    UserID, BevID

Upvotes: 0

Dan P
Dan P

Reputation: 1999

Why not include user_id in they daycounts view and group by user_id and date.

Also include user_id in view t.

Then when you are queering against t add the user_id to the where clause.

Then you don't have to recreate your views for every single user you just need to remember to include in your where clause.

Upvotes: 0

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