in the gorm official document "Model definition" section, the screenshot is shown as follows: 
  http://gorm.io/zh_CN/docs/mod.
  
  question:  
 1,  Age  field type is  sql.NullInt64 , what does this  sql.NullInt64  mean? Why should I use the pointer  * string  for the types of 
 2 and  MemberNumber  fields? Where does it point? 
 3,  Email  and  Role  fields are all of  string  type, followed by  type:varchar (100)  and  size:255 , respectively. Suppose  size  is also changed to  100 , then: 
type:varchar(100)
size:100what"s the difference between the two ways of writing?
