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Martin Martin - 10 months ago 42
MySQL Question

Selecting multiple columns/fields in MySQL subquery

Basically there is attribute table and translation table - many translations for one attribute.

I need to select id and value from translation for each attribute in specified language, even if there is no translation record in that language. Either i am missing some join technique or join (without involving language table) is not working here since following do not return attributes with non existing translations in specified language.

select a.attribute,, at.translation
from attribute a left join attributeTranslation at on
where al.language=1;

So i am using subqueries like this, problem here is making two subqueries to the same table with same parameters (feels like performance drain unless mysql groups those, which i doubt since it makes you do many similar subqueries)

select attribute,
(select id from attributeTranslation where and language=1),
(select translation from attributeTranslation where and language=1),
from attribute a;

I would like to be able to get id and translation from one query, so i concat columns and get the id from string later, which is at least making single subquery, but still not looking right.

select attribute,
(select concat(id,';',title)
from offerAttribute_language
where and _language=1
from offerAttribute a

So the question part.
Is there a way to get multiple columns from a single subquery or should i use two subqueries (mysql is smart enough to group them?) or is joining the following way to go:

[[attribute to language] to translation] (joining 3 tables seems like worse performance than subquery).


Yes, you can do this. The knack you need is the concept that there are two ways of getting tables out of the table server. One way is ..


The other way is

FROM (SELECT col as name1, col2 as name 2 FROM ...) B

Notice that the select clause and the parentheses around it are a table, a virtual table.

So, using your second code example (I am guessing at the columns you are hoping to retrieve here):

SELECT a.attr,, b.trans, b.lang
FROM attribute a
 SELECT AS id, at.translation AS trans, at.language AS lang, a.attribute
 FROM attributeTranslation at
) b ON ( = b.attribute AND b.lang = 1)

Notice that your real table attribute is the first table in this join, and that this virtual table I've called b is the second table.

This technique comes in especially handy when the virtual table is a summary table of some kind. e.g.

SELECT a.attr,, b.trans, b.lang, c.langcount
FROM attribute a
 SELECT AS id, at.translation AS trans, at.language AS lang, at.attribute
 FROM attributeTranslation at
) b ON ( = b.attribute AND b.lang = 1)
 SELECT count(*) AS langcount,  at.attribute
 FROM attributeTranslation at
 GROUP BY at.attribute
) c ON ( = c.attribute)

See how that goes? You've generated a virtual table c containing two columns, joined it to the other two, used one of the columns for the ON clause, and returned the other as a column in your result set.