Using a CROSS JOIN:-
SELECT
o.id_outlet,
s_main.periode,
o.branch,
count(msisdn)
FROM
(
SELECT DISTINCT SUBSTRING(date,1,7) AS periode
FROM sales
) s_main
CROSS JOIN outlet o
LEFT OUTER JOIN sales s
ON s_main.periode = SUBSTRING(s.date,1,7)
AND o.id_outlet = s.id_outlet
WHERE (o.STATUS LIKE 'STREET%')
GROUP BY s_main.periode, o.branch, o.id_outlet
If you have a table of dates then you can just use that rather than the sub query to get the dates (which also avoids the potential problem of not having a date in the results for a month where there has been zero sales for any outlet).
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