Checking sequence with SQL query.

Question

I have a table orders that keeps all order from all our stores. I wrote a query to check the sequence orders for each store. It looks like that.

select WebStoreID, min(webordernumber), max(webordernumber), count(webordernumber) 
from orders
where ordertype = 'WEB' 
group by WebStoreID

I can check it all orders are present with this query. web ordernumber is number from 1...n.

How can I write query to find missing orders without joining to temporary/different table?

Answer 1

I would make an auxiliary table of "all integers from 1 to n" (see http://www.sql-server-helper.com/functions/integer-table.aspx for some ways to make it with a SQL Server function, but since it's something you will need over and over I'd make it into a real table anyway, and with any SQL engine it's easy to make that, just once) then use a nested query, SELECT value FROM integers WHERE value NOT IN (SELECT webordernumber FROM orders) &c. Also see http://www.sqlmag.com/Article/ArticleID/99797/sql_server_99797.html for a problem similar to yours, "detecting gaps in a sequence of numbers".

Answer 2

If your database supports analytic functions then you could use a query something like:

select prev+1, curr-1 from
( select webordernumber curr,
         coalesce (lag(webordernumber) over (order by webordernumber), 0) prev
  from   orders
)
where prev != curr-1;

The output will show the gaps e.g.

prev+1 curr-1
------ ------
     3      7

would mean that numbers 3 to 7 inclusive are missing.

Answer 3

You could join the table on itself to detect rows which have no previous row:

select cur.*
from orders cur
left join orders prev 
    on cur.webordernumber = prev.webordernumber + 1
    and cur.webstoreid = prev.webstoreid
where cur.webordernumber <> 1
and prev.webordernumer is null

This would detect gaps in the 1...n sequence, but it would not detect duplicates.