Runs in groups

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This is the classic "runs" problem.

select category
     , mark
     , start
     , case when start = end 
            then null
            else end end as end
  from (
select dt.category
     , dt.mark
     , min(dt.num) as start
     , max(dt.num) as end
  from (
       select s1.category
            , s1.mark
            , s1.num
            , s1.num -
              ( select count(*)
                  from goods as s2
                 where s2.category = s1.category
                   and s2.mark     = s1.mark
                   and s2.num     <= s1.num )
               as diff_rank
        from goods as s1
      ) as dt
group
    by dt.category
     , dt.mark
     , dt.diff_rank
) as dt2
order
    by category
     , start

For the solution, I am indebted to Joe Celko's SQL for Smarties, 3rd edition, page 556. Please do not ask me to explain it. Instead, test it yourself, by building up your query starting at the DT subquery. Analyze the results carefully. The outermost query is merely for cosmetic purposes.

Incidentally, this is the first time I can recall ever writing ELSE END END AS END in SQL. This may not work in every database system, so use identifier delimiters as necessary.