SELECT 
    c.name AS city_name,
    SUM(s.value) AS sales_total
FROM
    city c
LEFT OUTER JOIN
    sales s
ON
(
    c.id = s.city_id                                  --  "id" ᫥,  祭 ᥣ 
    AND
    s.day >= DATE_SUB(NOW(), INTERVAL 1 MONTH)        -- ᫮ । 㯯஢ ("join condition")
)
GROUP BY
    c.id                                              -- 㯯஢  ࠬ   த
ORDER BY
    c.name                                            -- 㯮冷   த, "A..Z"
