The problem of gorm definition Model of golang

in the gorm official document "Model definition" section, the screenshot is shown as follows:
http://gorm.io/zh_CN/docs/mod.

clipboard.png

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:100

what"s the difference between the two ways of writing?

Feb.26,2022
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b3bf7e-40862.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b3bf7e-40862.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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