Question

I have a table with IDs, rank, chart_date, and pageviews. It’s based on a cron job that is run nightly and compiles the number of pageviews for that ID.

For instance:

ID   |   RANK   |   PAGEVIEWS   |  CHART_DATE
5          1           100         2012-10-14
9          2           75          2012-10-14
13         3           25          2012-10-14
9          1           123         2012-10-13
5          2           74          2012-10-13
19         3           13          2012-10-13

So I’m grabbing today’s chart based on 2012-10-14 and ranking the data by 1-3. But I also want to show the rank where the ID was on the previous date.

For instance, on 2012-10-14 ID 5 was ranked 1 but on 2012-10-13 it was ranked 2.

Can I do this with one query? Or do I have to loop thru the results based on today and do a query for each ID?

Answer 1

Can I do this with one query?

You can, but you need a JOIN between the table with today’s date and the table with yesterday’s date:

SELECT today.*, yesterday.rank
    FROM yourtable AS today
    JOIN yourtable AS yesterday
        ON (today.id = yesterday.id
            AND today.chart_date = date(now())
            AND yesterday.chart_date = date(date_sub(now(), interval 1 day))
        )
    ORDER BY today.rank DESC;

You can even show the difference:

SELECT today.*, yesterday.rank AS yest, yesterday.rank-today.rank AS incr
    FROM yourtable AS today
    LEFT JOIN yourtable AS yesterday
        ON (today.id = yesterday.id
            AND today.chart_date = date(now())
            AND yesterday.chart_date = date(date_sub(now(), interval 1 day))
        )
    ORDER BY today.rank DESC;

ID   |   RANK   |   PAGEVIEWS   |  CHART_DATE  |  YEST  |  INCR
5          1           100         2012-10-14     2     |   1
9          2           75          2012-10-14     1     |  -1
13         3           25          2012-10-14     4     |   1

(LEFT JOIN ensures today’s data is there even if yesterday’s isn’t).