Question

I’m trying to compare two values in the same table, and check if there is a difference.
Right now, I have 1485 records in the cms_statistics_pages table, but when the query below:

SELECT
    cp.identifier,
    COUNT(csp1.statID) AS hits,
    COUNT(csp2.statID) AS hits_yesterday,
    IF(COUNT(csp1.statID)>COUNT(csp2.statID),1,0) AS growth
FROM cms_pages cp
LEFT JOIN cms_statistics_pages csp1
      ON csp1.pageID = cp.pageID
      AND DATE(csp1.datetime) = '2012-07-20'
LEFT JOIN cms_statistics_pages csp2
      ON csp2.pageID = cp.pageID
      AND DATE(csp2.datetime) = '2012-07-19'
GROUP BY cp.identifier

..is fired, I get these results:

identifier                  hits    hits_yesterday  growth
index                       13395   13395           0
siden-er-under-opdatering   638     638             0
vores-historie              0       3               0

Which is not correct for my purpose. Then if I change:

AND DATE(csp1.datetime) = '2012-07-20'

to a date that will match no records

AND DATE(csp1.datetime) = '2012-07-21'

My result now looks like this:

identifier                  hits   hits_yesterday   growth
index                       0      141              0
siden-er-under-opdatering   0      29               0
vores-historie              0      3                0

Now the hits are correct, so I’m wondering if the query counts the records multiple times when both the joins contains some data.

Example data from cms_pages:

    pageID  sectionID   templateID  identifier  default title exclude_title 
    1       1       1           index   1       Welcome to SiteTech Framework 2012

Example data from cms_statistics_pages:

    statID  frontend    backend     pageID  sectionID   panel   datetime
    1       0       1       34      6       admin   2012-07-17 12:34:14

Answer 1

So I’ve been messing around a bit with the query, and found a solution which includes left joins and sub queries. My query now looks like:

SELECT 
    cp.identifier,
    now.hits AS hits,
    yd.hits AS hits_yesterday,
    IF(now.hits>yd.hits,1,0) AS growth
FROM cms_pages AS cp
LEFT JOIN 
    (
        SELECT
            pageID,
            COUNT(pageID) AS hits
        FROM cms_statistics_pages
        WHERE DATE(datetime) = '2012-07-20'
        GROUP BY pageID
    ) now
ON now.pageID = cp.pageID
LEFT JOIN
    (
        SELECT  pageID,
            COUNT(pageID) AS hits
        FROM cms_statistics_pages
        WHERE DATE(datetime) = '2012-07-19'
        GROUP BY pageID
    ) yd
ON yd.pageID = cp.pageID

Which gave me this correct result!:

identifier                  hits    hits_yesterday  growth
index                       95      141             0
siden-er-under-opdatering   22      29              0
vores-historie              NULL    3               0