MySQL访问上一行的值

Question

I have two columns Page_from and Page_to . When the records are sorted, they appear as...

Page_from   Page_to
1              4
5              7
9              11

Here page number 8 is missing. I want to find the missing page number.So I must be able to compare the value of Page_to in previous row with Page_from in current row.

Answer 1

You can find the beginning of a missing sequence by finding the previous record, and comparing the previous page_to and the current page_from . If there is a gap, you can get both the first and last page in the gap.

select tprev.page_to + 1 as missing_page_from, t.page_from - 1 as missing_page_to
from (select t.*,
             (select tprev.page_to
              from t tprev
              where tprev.page_from < t.page_from
              limit 1
             ) as prev_to
      from t
     ) t
where prev_to is not null and
      prev_to <> t.page_from - 1;