Select user activity over past months grouped by cohorts
With the help of Rick to look for pivot table I was able to create the final query as described in this post: https://stackoverflow.com/questions/7674786/mysql-pivot-table

Query for a 6 months analysis:

SELECT 
    YEAR(registered), MONTH(registered), COUNT(*) as total,
    SUM(lastLogin &gt;= DATE(registered) - INTERVAL DAYOFMONTH(registered) - 1 DAY + INTERVAL 1 MONTH) AS month1,
    SUM(lastLogin &gt;= DATE(registered) - INTERVAL DAYOFMONTH(registered) - 1 DAY + INTERVAL 2 MONTH) AS month2,
    SUM(lastLogin &gt;= DATE(registered) - INTERVAL DAYOFMONTH(registered) - 1 DAY + INTERVAL 3 MONTH) AS month3,
    SUM(lastLogin &gt;= DATE(registered) - INTERVAL DAYOFMONTH(registered) - 1 DAY + INTERVAL 4 MONTH) AS month4,
    SUM(lastLogin &gt;= DATE(registered) - INTERVAL DAYOFMONTH(registered) - 1 DAY + INTERVAL 5 MONTH) AS month5,
    SUM(lastLogin &gt;= DATE(registered) - INTERVAL DAYOFMONTH(registered) - 1 DAY + INTERVAL 6 MONTH) AS month6
FROM
    user
WHERE 
    registered &gt;= "2015-01-01"
GROUP BY YEAR(registered) , MONTH(registered)


The part DATE_SUB(DATE(registered), INTERVAL DAYOFMONTH(registered) - 1 DAY) will return the first of a month to make sure the analysis is based on calendar months. I am not sure if it could be simplified. I was looking at STR_TO_DATE(LEFT(registered, 7), "%y-%m") but I was reading working on Strings is slower and should be avoided. If someone knows a better / cleaner way of doing that, please comment and I will include it in the answer.

Here is some exemplary output of the query:

  year, month, total , month1, month2, month3, month4, month5, month6
'2015',  '1', '14776', '4302', '3225', '2827', '2547', '2330', '2163'
'2015',  '2', '12162', '3859', '2866', '2465', '2259', '2046', '1890'
'2015',  '3', '10770', '3831', '2841', '2507', '2288', '2135', '1987'
'2015',  '4', '15685', '3731', '2641', '2273', '2045', '1872', '1730'
'2015',  '5', '18130', '3686', '2403', '1993', '1744', '1528', '1363'