Adding null values for records not existing in joined table

Question

I have a table, called top_trends which has the following schema:

id int(11) AI PK 
criteria_id int(11) 
value varchar(255) 
created_at timestamp 
updated_at timestamp

Then I have another table, called search_criterias which has the following schema:

id int(11) AI PK 
title varchar(255) 
created_at timestamp 
updated_at timestamp

Here is a query which provides the value based on maximum number of records by a value. So, if theres 2 records in top_trends with both having criteria_id as 1 and both have values of 3 in the top_trends.value column, and then a separate SINGLE record with the same criteria_id 1 but a value of 2, the query will produce a result of the selected criteria (being 1) having a value of 3 since the value 3 occurred more times than any other rows of values with criteria_id 1. So, in simple terms, the query chose the value 3 for criteria id 1 because that occurred the most amount of times based on the records in the top_trends having criteria_id 1

select
  x.value as `values`
  , sc.id as id
  , sc.title
  , sc.created_at
  , sc.updated_at
  , x.criteria_id as search_category_id 
from
  ( 
    select
      criteria_id
      , `value` 
    from
      top_trends 
    group by
      `criteria_id` 
    order by
      `value`
  ) x 
  left join search_criterias sc 
    on sc.id = x.criteria_id 
group by
  criteria_id

My issue is that unfortunately right now we dont have data for all possible search_criterias so some of the records in the search_criteria table are not being aggregated in my query.

For example, we have a search_criteria record of city, with an id of 5, but no records in the top_trends table with having a criteria_id of 5... so the query above is not including that search_criteria.

What I'd like to do is include those records in the search_criteria table that are not in the top_trends table but have the values attribute as null

Answer 1

LEFT JOIN is used when there are rows in the left table that have no match in the right table. Since the missing rows in your case are in top_trends , that should be the right table of the LEFT JOIN . So switch the order of the join.

select
  x.value as `values`
  , sc.id as id
  , sc.title
  , sc.created_at
  , sc.updated_at
  , x.criteria_id as search_category_id 
from search_criterias sc 
left join
  ( 
    select
      criteria_id
      , `value` 
    from
      top_trends 
    group by
      `criteria_id` 
    order by
      `value`
  ) x 
    on sc.id = x.criteria_id 
group bysc.id