sql - Mysql group_concat() re-orders resultset -



sql - Mysql group_concat() re-orders resultset -

consider mysql schema this:

create table `translations` ( group_id int(11), lang varchar(2), text varchar(9), unique index `group_id_lang` (`group_id`, `lang`) )

it should quite obvious going on here - have many translations 1 group_id.

now if i'll run next query, groups preferred language 'en', fallback 'ru', , if neither of these translations exists, fallback available language, works charm

select * ( select * translations order lang = 'en' desc, lang = 'ru' desc ) translations grouping group_id +----------+------+---------+ | group_id | lang | text | +----------+------+---------+ | 1 | en | republic of estonia | | 2 | en | england | | 3 | ru | Швеция | +----------+------+---------+

as see i'll 3 rows lastly 1 fallen 'ru' because 'en' wasn't nowadays on group. far good..

now if want select available languages column adding group_concat(lang) translations, things bit fuzzy:

select *, group_concat(lang) available_translations ( select * translations order lang = 'en' desc, lang = 'ru' desc ) translations grouping group_id +----------+------+-----------+--------------+ | group_id | lang | text | translations | +----------+------+-----------+--------------+ | 1 | et | eesti | et,en,ru | | 2 | et | inglismaa | et,en,ru | | 3 | ru | Швеция | ru,et | +----------+------+-----------+--------------+

now mysql doesn't respect order by clause, , groups in native order there's no ordering @ all.

why group_concat causing issue?

my best guess mysql reordering result easier text grouping..

how can create work have preferred languages , list available translations row?

also, if you'd fiddle around here go

the order of results not guaranteed until outer query, though have ordered subquery, not guaranteed order records read, , returned outer query. main issue though, abusing mysql grouping extension allows select columns not contained in grouping clause or aggregate function.

to simplify query have:

select group_id, text translations grouping group_id order lang = 'en' desc, lang = 'ru' desc

unless text functionally dependant on group_id breaks sql standard, nonetheless, mysql allows it, however, stated in mysql documents

the server free take value each group, unless same, values chosen indeterminate. furthermore, selection of values each grouping cannot influenced adding order clause.

so though have order not apply until after 1 row per grouping has been selected, , 1 row non-determistic. doing ordering final result set.

in order work around , determinsitic results, can concatenate texts using group_concat , explicitly state required order:

group_concat(text order lang = 'en' desc, lang = 'ru' desc)

then utilize substring_index extract first term. query becomes:

select group_id, substring_index( group_concat(lang order lang = 'en' desc, lang = 'ru' desc), ',', 1) lang, substring_index( group_concat(text order lang = 'en' desc, lang = 'ru' desc), ',', 1) text, group_concat(lang order lang = 'en' desc, lang = 'ru' desc) available_translations translations grouping group_id;

example on sql fiddle

mysql sql group-by sql-order-by

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