1.在没有设置默认值的情况下：

SELECT userinfo.id, user_name, role, adm_regionid, region_name , create_time
FROM userinfo
LEFT JOIN region ON userinfo.adm_regionid = region.id

设置显示默认值：

SELECT userinfo.id, user_name, role, adm_regionid, IFNULL(region_name,0) as region_name, create_time
FROM userinfo
LEFT JOIN region ON userinfo.adm_regionid = region.id