Re: [GENERAL] select and join

Morelli 'ZioBudda' Davide Michel wrote:

> My question is: there is a way to join the table "utente" so to make a
> "select" that return me the column "utente.cognome",too ?
>
> i have make this try:
>
> esame=> select utente.cognome,
> prestito.id_utente,
> libro.tipo,
> count(*)
> from prestito, libro
> where libro.id_libro = prestito.id_libro and
> utente.id_utente = prestito.id_utente
> group by id_utente,
> tipo\g

(re-arranged)

Generally, the column functions/aggregates like count() can be applied
only if all non-aggregates/functions appear in the "group by"-section;
you joined the three tables correctly (however, you forgot to put the
third table name into the "from"-section!), but you have to tell the
database engine to determine the count grouped by "utente.cognome",
too. Try:

select utente.cognome,
prestito.id_utente,
libro.tipo,
count(*)
from utente, prestito, libro
where libro.id_libro = prestito.id_libro and
utente.id_utente = prestito.id_utente
group by utente.cognome,
prestito.id_utente,
libro.tipo

The result should be what you desired, because the relation
"utente.cognome"->"prestito.id_utente" is one-to-one. And you should
specify the table name for "id_utente" in the "group by"-section, too,
because this column name is not unique. Anyway, I did not test
anything, and maybe I overlooked something important ... I hope it
helps ...

Yours, Ulf

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