MySQL/MariaDB/SQLite: DISTINCT CONCAT - mysql

Question

This question isn't necessarily just Laravel related, but I'm trying to fetch records, which are distinct by concatenated fields. I need this to work with both MySQL/MariaDB as well as SQLite for testing purposes.
While doing my research, I've found out that SQLite does not have CONCAT function - instead you're using || operator to concatenate items. MySQL on the other hand will not interpret || the same way, but I can always use the conditional statement just to cover both cases.
However, I still cannot get records I'm after - my table consists of:
| id | tagable_id | tagable_type | name | title | description | url | image | hits |
| 1 | 1 | App\Models\Article | a.. | A.. | A.. descr.. | https://localhost | https://localhost... | 0 |
| 2 | 1 | App\Models\Article | b.. | B.. | B.. descr.. | https://localhost | https://localhost... | 2 |
| 3 | 1 | App\Models\Article | c.. | C.. | C.. descr.. | https://localhost | https://localhost... | 3 |
| 4 | 1 | App\Models\Page | a.. | A.. | C.. descr.. | https://localhost | https://localhost... | 0 |
I need get only 4 records that are sorted ASC by number of hits and which are unique using CONCAT(table_id, tagable_type).
What the statement should return in this case would be records with id 1 and 4 - because 2 and 3 have the same tagable_id and tagable_type as record with id 1, which has lowest number of hits - effectively only returning only 2 records:
| id | tagable_id | tagable_type | name | title | description | url | image | hits |
| 1 | 1 | App\Models\Article | a.. | A.. | A.. descr.. | https://localhost | https://localhost... | 0 |
| 4 | 1 | App\Models\Page | a.. | A.. | C.. descr.. | https://localhost | https://localhost... | 0 |
I tried already:
DB::table('tags')
->selectRaw("DISTINCT CONCAT(`tagable_id`, '-', `tagable_type`), `id`, `name`, `title`, `description`, `url`, `image`")
->whereIn('name', $tags->toArray())
->orderBy('hits');
This however does not return distinct records - it will return all records regardless of the distinct concatenation - that is in MySQL / MariaDB - in SQLite it will tell me no such function: CONCAT.
I also tried:
DB::table('tags')
->selectRaw("CONCAT(`tagable_id`, '-', `tagable_type`) as `identifier`, `id`, `name`, `title`, `description`, `url`, `image`")
->whereIn('name', $tags->toArray())
->groupBy('identifier')
->orderBy('hits');
This time MySQL/MariaDB tells me that I need to include other fields in the group by as well tags.id' isn't in GROUP BY, but when I use it with SQLite and replace CONCAT function with (tagable_id || '-' || tagable_type) as identifier - it seem to work.
So at this stage I'm: MySQL: 0 | SQLite: 1
Any help would be much appreciated.
UPDATE
After hours of trying to get it resolved I've decided to add a new column
identifier to the table to overcome issue of the non available concat function - my code now looks like this:
Tag::with('tagable')->whereIn('id', function($query) use ($tags) {
$query->selectRaw('min(`id`) from `tags`')
->whereIn('name', $tags->toArray())
->groupBy('identifier');
})
->orderBy('hits')
->take(4)
->get();
This is still not quite what I'm after as it relies on the lowest id min(id) of the given identifier and if the record with the lowest id for the same identifier has higher number of hits then its sibling then the sibling will not be returned.

Answer 1

You can use DISTINCT to get distinct combinations, but not whole rows:
DB::table('tags')->distinct()->get(['tagable_id', 'tagable_type']);
You have to use a more complex query for that:
$join = DB::table('tags')
->select('tagable_id', 'tagable_type')->selectRaw('MIN(hits) as hits')
->whereIn('name', $tags->toArray())
->groupBy('tagable_id', 'tagable_type');
$sql = '(' . $join->toSql() . ') as grouped';
$tags = DB::table('tags')
->join(DB::raw($sql), function($join) {
$join->on('tags.tagable_id', '=', 'grouped.tagable_id')
->on('tags.tagable_type', '=', 'grouped.tagable_type')
->on('tags.hits', '=', 'grouped.hits');
})->mergeBindings($join)
->whereIn('name', $tags->toArray())
->get();
A solution that guarantees one record per unique combination:
$join = DB::table('tags')
->select('tagable_id', 'tagable_type')->selectRaw('MIN(hits) as hits')
->whereIn('name', $tags->toArray())
->groupBy('tagable_id', 'tagable_type');
$sql = '(' . $join->toSql() . ') as grouped';
$ids = DB::table('tags')
->selectRaw('MIN(id) as id')
->join(DB::raw($sql), function($join) {
$join->on('tags.tagable_id', '=', 'grouped.tagable_id')
->on('tags.tagable_type', '=', 'grouped.tagable_type')
->on('tags.hits', '=', 'grouped.hits');
})->mergeBindings($join)
->whereIn('name', $tags->toArray())
->groupBy('tags.tagable_id', 'tags.tagable_type')
->pluck('id');
$tags = DB::table('tags')->whereIn('id', $ids)->get();