1 表数据person_info
id name age
1 qq 23
2 ee null
3 rr 8
2 按照年龄排序：
（1）（升序，null在最后）
select * from person_info order by age; 等同 select * from person_info order by age asc;
id name age
3 rr 8
1 qq 23
2 ee null
（2）（降序，null在最前）
select * from person_info order by age desc;
id name age
2 ee null
1 qq 23
3 rr 8
（3）（升序，null在最前）
select * from person_info order by age nulls first;
id name age
2 ee null
3 rr 8
1 qq 23
（4）（降序，null在最后）
select * from person_info order by age desc nulls end;
id name age
1 qq 23
3 rr 8
2 ee null