-- 语法：update table_1 t1,table_2 t2 set t1.column = t2.column where t1.id = t2.pid;

UPDATE house_test.shareInfo AS t1,

(

    SELECT

        u.*,

        g.city_code

    FROM

        `api`.`user` AS u

        LEFT JOIN `api`.`group` AS g ON g.group_id = u.group_id

    ) AS t2

    SET t1.cityCode = t2.city_code

WHERE

    t1.userId = t2.user_id

    AND (

        t1.cityCode IS NULL

    OR t1.cityCode = '')