1.在沒有設置默認值的情況下：
SELECT userinfo.id, user_name, role, adm_regionid, region_name , create_time FROM userinfo LEFT JOIN region ON userinfo.adm_regionid = region.id結果：
設置顯示默認值：
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即：如果region_name一列的值為空，則顯示為0，結果為：
