Add sequence record numbers

Question

I have a table with with two fields: ID field (FID) as primary key and Qty both are integers. I need to query this table and add two additional fields as sequence serial using the QTY field so I can get results like this:

table1
-------------------------------------
ID   Qty   range_begin   range_end
50   2         1           2
53   1         3           3
65   3         4           6
67   2         7           8     

range_begin field start at 1, next record should be range_end + 1. And range_end = last range_end + Qty

I tried to write it using context variables Like this

I set all variables to null first

rdb$set_context('USER_TRANSACTION', 'range_end', null);
rdb$set_context('USER_TRANSACTION', 'range_begin', null);

Than I start the query:

Select 
  rdb$get_context('USER_TRANSACTION', 'range_begin'),
  rdb$set_context('USER_TRANSACTION', 'range_begin', COALESCE(CAST(rdb$get_context('USER_TRANSACTION', 'range_end') AS INTEGER), 0) + 1)),

  rdb$get_context('USER_TRANSACTION', 'range_end'),
  rdb$set_context('USER_TRANSACTION', 'range_end', COALESCE(CAST(rdb$get_context('USER_TRANSACTION', 'range_end') AS INTEGER), 0) + Qty)),

  Qty
  from table1 where ... 

But I did not able to get the correct sequence. I also tried to add more variables and other things but none worked for me so how this should be done so I can use single select query ?

I am using FireBird 2.5 and it is not necessary for the answer to use context variables.

Answer 1

Basically, what you want is

SELECT t.ID, t.QTY, 
  ((select coalesce(sum(qty),0) from table1 a where a.ID < t.ID) + 1) as RANGE_BEGIN,
  ((select coalesce(sum(qty),0) from table1 a where a.ID < t.ID) + t.qty) as RANGE_END
FROM table1 t

For a big table this might get slow, so instead of calculated field you might want to use permanent fields and calculate the range values in trigger... it depend how the data changes etc.