请教个查询的问题，希望兄弟们不吝指教下

mysql> Select sum(TotalUser) from UserStat where Year >= 2015 and Month >= 09 and Day >= 28 limit 10;
+----------------+
| sum(TotalUser) |
+----------------+
| 110 |
+----------------+
1 row in set
mysql> Select sum(TotalUser) from UserStat where Year <= 2015 and Month <= 10 and Day <= 05 limit 10;
+----------------+
| sum(TotalUser) |
+----------------+
| 150 |
+----------------+
1 row in set
//想请教下为什么这条查不到数据
mysql> Select sum(TotalUser) from UserStat where Year >= 2015 and Month >= 09 and Day >= 28 and Year <= 2015 and Month <= 10 and Day <= 05 limit 10;
+----------------+
| sum(TotalUser) |
+----------------+
| NULL |
+----------------+
1 row in set