if 100=(SELECT COUNT(*) FROM userlist)
select * from userlist where ID=(select max(ID) FROM userlist)
insert userlist select id,username ---id,username 由你定
select * from userlist where ID=(select max(ID) FROM userlist)
insert userlist select id,username ---id,username 由你定
delete userlist where ID=(select max(ID) FROM userlist)
insert userlist select id,username ---id,username 由你定
或
delete userlist
where ID=(select max(ID) FROM userlist) and 100=(SELECT COUNT(*) FROM userlist)
insert userlist select id,username ---id,username 由你定
能不能给个例子?比如用户abcd,他的ID号1000
或者 insert into userlist(id,username) values(1000,'abcd')
delete userlist where ID=(select max(ID) FROM userlist)
insert userlist select id,username