I'm working on a ROR application and i use Hstore with PostgreSQL data base and i'm asking if i can do group by an Hstore key on my table ??
I did this :
select state , count(*) from infractions group by details -> 'commune';`
but i get this error :
column "infractions.state" must appear in the GROUP BY clause or be used in an aggregate function
details is the Hstore column and her is an example of my details :
"adress"=>"", "commune"=>"14", "province"=>"6", "description"=>"Ce fichier est sous licence Creative Commons"
thanks
People run into this occasionally when they move from MySQL. MySQL allows columns to be omitted from the GROUP BY clause. PostgreSQL does not. Your actual issue is that you are missing the state column. You have two options. If you want to pull just one state you can put it in an aggregate like this:
select max(state) , count(*) from infractions group by details -> 'commune';
If you want to group by state too you can:
select state , count(*) from infractions group by state, details -> 'commune';
Related
I have this table:
User
Name
Role
Mason
Engineer
Jackson
Engineer
Mason
Supervisor
Jackson
Supervisor
Graham
Engineer
Graham
Engineer
There can be exact duplicates (same Name/Role combination). Ignore comments about primary key.
I am writing a query that will give the distinct values from 'Name' column, with the corresponding 'Role'. To select the corresponding 'Role', if there is a 'Supervisor' role for a name, that record is returned. Otherwise, a record with the 'Engineer' role should be returned if it exists.
For the above table, the expected result is:
Name
Role
Mason
Supervisor
Jackson
Supervisor
Graham
Engineer
I tried ordering 'Role' in descending order, so that I can group by Name,Role and pick the first item - it will be a 'Supervisor' role if present, else 'Engineer' role - which matches my expecation.
I also tried doing User.select('DISTINCT ON (name) \*).order(Role: :desc) - I am not seeing this clause in the SQL query that gets executed.
Also, I tried another approach to get all valid Name, Role combinations and then process it offline iterating the result set and using if-else to decide which row to display.
However, I am interested in anything that is efficient and does not over do this handling.
I am new to Ruby and therefore reaching out.
If I wanted to do this in pure SQL, I would have to use GROUP BY.
SELECT Name, MAX(Role) FROM User GROUP BY Name
So one method would be to execute this SQL statement against the base connection.
ActiveRecord::Base.connection.execute("SELECT Name, MAX(Role) FROM User GROUP BY Name")
That would provide exactly the data you need, though it wouldn't be returned as ActiveRecord models. If you need those models then I would use find_by_sql and do an inner join to provide the records.
User.find_by_sql("SELECT User.* FROM User INNER JOIN (SELECT Name AS n, MAX(Role) AS r FROM User GROUP BY Name) U2 WHERE Name = U2.n AND Role = U2.r")
Unfortunately that would provide both records for Graham.
I am trying to order all the recipes in my database by the number of likes they have received. Likes are polymorphic and belong to :likeable while a recipe has many likes.
My query works for SQLite3, but when I upload to Heroku using PostgreSQL it seems to break things.
function is as follows:
Recipe.select('*').joins(:likes).group('recipes.id').order('COUNT(likes.likeable_id)')
And the error that Heroku gives me when I try to run the website:
ActionView::Template::Error (PG::GroupingError: ERROR: column "likes.id" must appear in the GROUP BY clause or be used in an aggregate function
Everything compiles, but the homepage uses that scope function so I get a server error right away.
You need to explicitly select recipies.*:
Recipe.select(
Recipe.arel_table[:*],
Likes.arel_table[:*].count.as('likes_count')
)
.joins(:likes)
.group(:id)
.order(:likes_count)
Selecting the count is really optional - you can skip .select entirely and just fetch the aggregate in the order clause:
Recipe.joins(:likes)
.group(:id)
.order(Likes.arel_table[:*].count)
You cannot select * from grouping by.
for most SQL-dabases (Postgres, newer Mysql, ...) you can only use SELET columns in a GROUP BY:
columns you've grouped by, and that are transient by the grouped column (e.g. grouping recipes.id can also select recipes.title)
And aggregated columns (count, sum, max)
Try:
Recipe.select('recipies.*').joins(:likes).group(:id).order('COUNT(likes.likeable_id)')
I am using Model.group(:category) in order to get unique records based on category field in Rails 5 application.
Table data:
id catgeory description
1 abc test
2 abc test1
3 abc test2
4 xyz test
5 xyz testabc
I want records (1,4) as a result. Therefore I am using Model.group(:category) which works fine for MYSQL whose sql_mode is " " .
Unforunately its throwing an error "SELECT list is not in GROUP BY clause and contains nonaggregated column which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by" whose sql_mode is "only_full_group_by".
whats the best way to change the query to match the mode?
Perhaps try specifying which id you want? You could use MIN(id), MAX(id) etc.
MySQL supports a non-standard extension to SQL described here. To continue using that behavior, you could change the sql_mode to TRADITIONAL in config/database.yml.
This ActiveRecord query works in SQLite:
SlotReq.group(:team_id)
In PostgreSQL, the same query gives this error "GroupingError - column slot_reqs.id must appear in the GROUP BY clause or be used in an aggregate function"
Based on the answer to this question I changed my query to:
SlotReq.select("slot_reqs.team_id").group("slot_reqs.team_id")
and it works as expected.
I would like to know if I'm doing it right and why does this work?
Yes, you are doing it right, although you could also use:
SlotReq.select(:team_id).group(:team_id)
What happens is that PG (among other DB's) needs that all column names in SELECT must be either aggregated or used in GROUP BY clause; this is because, if any unagreggated column is omitted, it could lead to indeterminate behavior (i.e. What value should be used in that column?)
So, by specifying in select just the column you want to group, you don't omit any column; on the other hand, using group withoutselect, is equivalent to doing SELECT * FROM table GROUP BY column, which brings all columns but only one being specified on GROUP BY clause.
I am getting this error in the pg production mode, but its working fine in sqlite3 development mode.
ActiveRecord::StatementInvalid in ManagementController#index
PG::Error: ERROR: column "estates.id" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: SELECT "estates".* FROM "estates" WHERE "estates"."Mgmt" = ...
^
: SELECT "estates".* FROM "estates" WHERE "estates"."Mgmt" = 'Mazzey' GROUP BY user_id
#myestate = Estate.where(:Mgmt => current_user.Company).group(:user_id).all
If user_id is the PRIMARY KEY then you need to upgrade PostgreSQL; newer versions will correctly handle grouping by the primary key.
If user_id is neither unique nor the primary key for the 'estates' relation in question, then this query doesn't make much sense, since PostgreSQL has no way to know which value to return for each column of estates where multiple rows share the same user_id. You must use an aggregate function that expresses what you want, like min, max, avg, string_agg, array_agg, etc or add the column(s) of interest to the GROUP BY.
Alternately you can rephrase the query to use DISTINCT ON and an ORDER BY if you really do want to pick a somewhat arbitrary row, though I really doubt it's possible to express that via ActiveRecord.
Some databases - including SQLite and MySQL - will just pick an arbitrary row. This is considered incorrect and unsafe by the PostgreSQL team, so PostgreSQL follows the SQL standard and considers such queries to be errors.
If you have:
col1 col2
fred 42
bob 9
fred 44
fred 99
and you do:
SELECT col1, col2 FROM mytable GROUP BY col1;
then it's obvious that you should get the row:
bob 9
but what about the result for fred? There is no single correct answer to pick, so the database will refuse to execute such unsafe queries. If you wanted the greatest col2 for any col1 you'd use the max aggregate:
SELECT col1, max(col2) AS max_col2 FROM mytable GROUP BY col1;
I recently moved from MySQL to PostgreSQL and encountered the same issue. Just for reference, the best approach I've found is to use DISTINCT ON as suggested in this SO answer:
Elegant PostgreSQL Group by for Ruby on Rails / ActiveRecord
This will let you get one record for each unique value in your chosen column that matches the other query conditions:
MyModel.where(:some_col => value).select("DISTINCT ON (unique_col) *")
I prefer DISTINCT ON because I can still get all the other column values in the row. DISTINCT alone will only return the value of that specific column.
After often receiving the error myself I realised that Rails (I am using rails 4) automatically adds an 'order by id' at the end of your grouping query. This often results in the error above. So make sure you append your own .order(:group_by_column) at the end of your Rails query. Hence you will have something like this:
#problems = Problem.select('problems.username, sum(problems.weight) as weight_sum').group('problems.username').order('problems.username')
#myestate1 = Estate.where(:Mgmt => current_user.Company)
#myestate = #myestate1.select("DISTINCT(user_id)")
this is what I did.