Question

Here’s the thing. I’m having 3 tables, and I’m doing this query:

select t.nomefile, t.tipo_nome, t.ordine
from
(select nomefile, tipo_nome, categorie.ordine
from t_gallerie_immagini_new as immagini
join t_gallerie_new as collezioni on collezioni.id=immagini.id_ref
join t_gallerie_tipi as categorie on collezioni.type=categorie.id
order by RAND()
) as t
group by t.tipo_nome
order by t.ordine

It’s applied to 3 tables, all in relationship 1-N, which need to be joined and then take 1 random result from each different result in the higher level table. This query works just fine, the problem is that I’m being asked to rewrite this query USING ONLY ONE SELECT. I’ve come with another way of doing this with only one select, the thing is that according to SQL sintax the GROUP BY must be before the ORDER BY, so it’s pointless to order by random when you already have only the first record for each value in the higher level table.

Someone has a clue on how to write this query using only one select?

Answer 1

I was not able to understand the reasons behind the request to rewrite this query, however, i found out that there is a solution which uses the “select” word only once. Here’s the query:

SELECT g.type, SUBSTRING_INDEX(GROUP_CONCAT(
i.nomefile ORDER BY
RAND()),',',1) nomefile
FROM t_gallerie_new g JOIN t_gallerie_immagini_new i ON g.id=i.id_ref
GROUP BY g.type;

for anyone interested in this question.

NOTE: The use of GROUP_CONCAT has a couple of downsides: It is not recommended to use this keyword when using medium/large tables since it could increase the server side payload. Also, there is a limit to the size of the string returned by GROUP_CONTACT, by default 1024, so, it’s necessary to modify a parameter in the mySql server to be able to receive a bigger string from this instruction.