???????????????????????????????????????????????????е???????????????????????????????С??????????????????????????
???????磺???????user_t ??????user_name; user_age; add_time???????Σ?
???????????student_t??????student_name; student_age; add_time?????
???????????????????????????????е??????
?????????????
????select user_name as name?? user_age as age ??add_time as time from user_t
????union
????select student_name as name?? student_age as age ??add_time as time from user_t
????order by time desc(asc);
???????и??????????????????????????дok???