修改delete相关代码逻辑，新增README_students.md用来做模块解释。

students/model/students_model.py

@@ -3,7 +3,7 @@ import datetime
 
 from students.databases import *
 from datetime import datetime
-
+from sqlalchemy.dialects.mysql import DATETIME
 
 class Students(Base):
     __tablename__ = 'students'
@@ -27,11 +27,11 @@ class Students(Base):
 
     salary = Column(Integer , comment='就业薪资')
 
-    create_date = Column(DATETIME , default=datetime.now , comment='创建时间' )
+    create_date = Column(DATETIME(fsp=3) , default=datetime.now , comment='创建时间' )
 
-    update_date = Column(DATETIME , default=datetime.now , comment='更新时间' )
+    update_date = Column(DATETIME(fsp=3) , default=datetime.now , comment='更新时间' )
 
     is_deleted = Column(INTEGER , default=False , comment='是否删除：0-正常，1-已删除' )
 
-    deleted_date = Column(DATETIME , default=datetime.now , comment='删除时间' )
+    deleted_date = Column(DATETIME(fsp=3) , default=None , comment='删除时间'  )