How to combine near same item by SQL?

Question

I have some data in database:

id user 
1 zhangsan 
2 zhangsan 
3 zhangsan 
4 lisi 
5 lisi 
6 lisi 
7 zhangsan 
8 zhangsan 

I want keep order, a

Answer 1

if in sql server, oracle, db2...

    with x as(  
select c.*, rn = row_number() over (order by c.id)
  from test c 
  left join test n
    on c.[user] = n.[user]
   and c.[id] + 1 = n.[id]   
 where n.id is null
)
select a.[user], a.id - coalesce(b.id, 0) 
  from x a
  left join x b
    on a.rn = b.rn + 1