Re: how to "enumerate" rows ?

Karsten Hilbert <Karsten(dot)Hilbert(at)gmx(dot)net> writes:
>> Now, I need to enumerate the vaccinations per patient per
>> disease. They are intrinsically ordered by their date of
>> vaccination but I need to attach a number to them ...

The best, recommended way to do this is to plaster on the row numbers
in your client-side code. AFAIK there just isn't any way to do it in
standard SQL.

If you feel you really gotta have a server-side solution, the easiest
way is

CREATE TEMP SEQUENCE myseq;

SELECT nextval('myseq'), *
FROM (SELECT ... ORDER BY date_given) ss;

DROP SEQUENCE myseq;

(Alternatively, you can create a temp sequence once per session and just
reset it with setval() for each query.)

Note that you can *not* simplify this to

SELECT nextval('myseq'), ... ORDER BY date_given;

because if you do, the nextval() values may get computed before the
ORDER BY sorting occurs. Putting the ORDER BY into a sub-select makes
it work reliably.

This is not real portable since neither sequences nor ORDER BY in a
subselect are SQL-standard. It's also probably a good deal slower than
a simple client-side counter, because nextval() does a lot more than
just add one to a variable. But if you have to have it, there it is.

regards, tom lane