Question : Query to display missing sequence numbers/range for separate groups/subgroups

Hello,

I have been struggling with this query for a while now. Table structure is:

Table A : Aid, sequenceNumber, CategoryId, SubCategoryId
TableCategory: CategoryId, CategoryName
TableSubCategory: SubCategoryId, SubCategoryName

For each subcategory the sequence number will reset as well as for each category so for example:
For Category A, subcategory A it will be 1,2,3...n
For Category A, subcategory B it will be 1,2,3...n  
For Category B, subcategory A it will be 1,2,3...n and so on . It will always start from 1 to whatever.

I would like to write a query to find missing values in a sequence which is grouped by category and by subcategory as well as gaps - 'from', 'to' values

So for example, I should be able to tell values 2,3 and 4 through 8 are missing in Category A, subcategory B and so forth... I was able to write it without groupings but I am not sure how to handle 2 tier grouping and make sure each group and subgroup have the correct sequence order.

Let me know if I am not clear and any help would be appreciated.

Answer : Query to display missing sequence numbers/range for separate groups/subgroups

As ranges instead of individual records

select A.CategoryId, A.SubCategoryId, isnull((
      select top(1) c.sequenceNumber from TableA C
      where A.CategoryId=C.CategoryId and A.SubCategoryId=C.SubCategoryId
      and c.sequenceNumber<A.sequenceNumber
      order by c.sequenceNumber desc
      )+1,1) [From], A.sequenceNumber-1 [To]
from TableA A
where A.sequenceNumber>1 and not exists (
      select * from TableA B
      where A.CategoryId=B.CategoryId and A.SubCategoryId=B.SubCategoryId
      and B.sequenceNumber=A.sequenceNumber -1)