How can I select adjacent rows to an arbitrary row (in sql or postgresql)?

Question

I want to select some rows based on certain criteria, and then take one entry from that set and the 5 rows before it and after it.

Now, I can do this numerically if ther

Answer 1

If you wanted to do it in a 'relationally pure' way, you could write a query that sorted and numbered the rows. Like:

select (
  select count(*) from employees b
  where b.name < a.name
) as idx, name
from employees a
order by name

Then use that as a common table expression. Write a select which filters it down to the rows you're interested in, then join it back onto itself using a criterion that the index of the right-hand copy of the table is no more than k larger or smaller than the index of the row on the left. Project over just the rows on the right. Like:

with numbered_emps as (
  select (
    select count(*)
    from employees b
    where b.name < a.name
  ) as idx, name
  from employees a
  order by name
)
select b.*
from numbered_emps a, numbered_emps b
where a.name like '% Smith' -- this is your main selection criterion
and ((b.idx - a.idx) between -5 and 5) -- this is your adjacency fuzzy-join criterion

What could be simpler!

I'd imagine the row-number based solutions will be faster, though.