Project-3 Casestudy-2

SELECT DATEFORMAT(occurredat, '%Y-%U') AS weekstart,device, COUNT(DISTINCT userid) AS activeusers FROM events WHERE eventtype = 'engagement' GROUP BY weekstart, device ORDER BY weekstart, device; Im getting week as null values NULL, 'acer aspire desktop', '198' NULL, 'acer aspire notebook', '338' NULL, 'amazon fire phone', '89' NULL, 'asus chromebook', '355' NULL, 'dell inspiron desktop', '360' NULL, 'dell inspiron notebook', '677' NULL, 'hp pavilion desktop', '339' NULL, 'htc one', '196' NULL, 'ipad air', '478' NULL, 'ipad mini', '292' NULL, 'iphone 4s', '409' NULL, 'iphone 5', '1025' NULL, 'iphone 5s', '626' NULL, 'kindle fire', '205' NULL, 'lenovo thinkpad', '1309' NULL, 'mac mini', '150' NULL, 'macbook air', '950' NULL, 'macbook pro', '1952' NULL, 'nexus 10', '273' NULL, 'nexus 5', '621' NULL, 'nexus 7', '355' NULL, 'nokia lumia 635', '211' NULL, 'samsumg galaxy tablet', '107' NULL, 'samsung galaxy note', '119' NULL, 'samsung galaxy s4', '803' NULL, 'windows surface', '182'