Im trying to eliminate classes with only 1 pupil
Here is the result i have
Desired Result
I want only class c to show as it has more than one pupil
I believe its some way of using HAVING but i cant figure it out.
SELECT Pupil, class
FROM school
Using IN
select *
from school
where class in (select class
from school
group by class
having count(distinct Pupil) > 1)
Related
Problem:
I am using the ransack gem to sort columns in a table. I have 2 models: Campaign and Course. A campaign has many courses, and a course belongs to one campaign. Each course has a number of total_attendees. My Campaigns table has a column for Total Attendees, and I want it to be sortable. So it would sum up the total_attendees field for each course that belongs to a single campaign, and sort based on that sum.
Ex. A campaign has 3 courses, each with 10 attendees. The Total Attendees column on the campaign table would show 30 and it would be sortable against total attendees for all the other campaigns.
I found ransackers:
https://github.com/activerecord-hackery/ransack/wiki/Using-Ransackers
and this SO question: Ransack sort by sum of relation
and from that put together a lot of what is below.
From Model - campaign.rb:
class Campaign < ApplicationRecord
has_many :courses
ransacker :sum_of_total_attendees do
query = "SELECT SUM(r.total_attendees)
FROM campaigns c
LEFT OUTER JOIN courses r
ON r.campaign_id = c.id
GROUP BY c.id"
Arel.sql(query)
end
end
From Model - course.rb:
class Course < ApplicationRecord
belongs_to :campaign, optional: true
end
View:
<th scope="col"><%= sort_link(#q, :sum_of_total_attendees, 'Total Attendees') %></th>
Controller - campaigns_controller.rb:
all_campaigns = Campaign.all
#q = all_campaigns.ransack(params[:q])
#campaigns = #q.result
Errors:
The ransacker query gives me the data I want, but I don't know what to do to get the right information .
Originally, when I clicked on the th link to sort the data, I got this error:
PG::CardinalityViolation: ERROR: more than one row returned by a
subquery used as an expression
I don't know what changed, but now I'm getting this error:
PG::SyntaxError: ERROR: syntax error at or near "SELECT"
LINE 1: SELECT "campaigns".* FROM "campaigns" ORDER BY SELECT SUM(r....
^
: SELECT "campaigns".* FROM "campaigns" ORDER BY SELECT
SUM(r.total_attendees)
FROM campaigns c
LEFT OUTER JOIN courses r
ON r.campaign_id = c.id
GROUP BY c.id ASC
This error seems to say that the ransack search parameter, #q and the ransacker query don't work together. There are two selects in this request, when there should definitely be only one, but the first one is coming from ransack, so I'm not sure how to address it.
How do I get my query to sort correctly with ransack?
Articles I've looked at but did not seem to apply to what I was looking to accomplish with this story:
Ransack Sort By Sum of Relation: This is the one I worked from a lot, but I'm not sure why it works for this user and not for me. They don't show what is changed, if anything, in the controller
Ransack Github Issue For Multiple Params: This doesn't cover the issue of summing table columns.
Rails Ransack Sorting Searching Based On A Definition In The Model: This didn't apply to my need to sort based on summed data.
Three Ways to Bend The Ransack Gem: This looks like what I was doing, but I'm not sure why theirs is working but mine isn't.
I have 3 Models: Offer, Request and Assignment. Assignment makes a connection between Request and Offer. Now I want to do this:
select *
from offer as a
where places > (
select count(*)
from assignment
where offer_id = a.id and
to_date > "2014-07-07");
I am not quiet sure how to achieve this with a django QuerySet... Any tips?
Edit: The query above is just an example, how the query in general should look like. The django model looks like this:
class Offer(models.Model):
...
places = models.IntegerField()
...
class Request(models.Model):
...
class Assignment(models.Model):
from_date = models.DateField()
to_data = models.DateField()
request = models.ForeignKey("Request",related_name="assignments")
offer = models.ForeignKey("Offer",related_name="assignments")
People now can create a offer with a given amount of places or a request. The admin then will connect a request with an offer for a given time. This is saved as an assignment. The query above should give me a list of offers, which have still places left. Therefore I want to count the number of valid assignments for a given offer to compare it with its number of places. This list should be used to find a possible offer for a given request to create a new assignment.
I hope this describes the problem better.
Unfortunately related subqueries aren't directly supported by ORM operations. Usage of .extra(where=...) should be possible in this case.
To get the same results without using a subquery something like the following should work:
Offer.objects.filter(
assignment__to_date__gt=thedate
).annotate(
assignment_cnt=Count('assignment')
).filter(
assignment_cnt__lte=F('places')
)
The exact query depends on the model definitions.
query = '''select *
from yourapp_offer as a
where places > (
select count(*)
from yourapp_assignment
where offer_id = a.id and
to_date > "2014-07-07");'''
offers = Offer.objects.raw(query):
https://docs.djangoproject.com/en/1.6/topics/db/sql/
Using these tables:
*student {'s_id','s_name,'...} , class {'c_id','c_name',...} student2class {'s_id','c_id'}, grades {'s_id','c_id','grade'}*
Is it possible to perform a query (nested query?) put class name as subtitle and then all students (of that class) and grades, next class name as subtitle ...
The result I need is:
Maths
John .... C
Anna .... B
[...]
Biology
Anna .... C
Jack .... A
[...]
For each row from class I'll have a subquery fetching all data related with this class
No need of any sub-query. You can get your data this way:
SELECT c_name, s_name_, grade
FROM student, class, grades
WHERE student.s_id = grades.s_id and class.c_id = grades.c_id
ORDER BY c_name
The presentation of results depends on your system/tools, as others already said. This is a link to the solution for Microsoft Access:
http://office.microsoft.com/en-001/access-help/create-a-grouped-or-summary-report-HA001159160.aspx
The solution should be implemented in your client side code and not in the database. From database you should just get a simple table formatted data (subject, student, grade)
Then convert the above recordset to the format you want:
For an example in C# you could convert the recordset into lookup
var Lookup = DataSet.Tables[0].Rows.ToLookup(x => x["subjectColumn"]);
now you can loop through the lookup and format your result
foreach (var grade in Lookup)
{
subject = grade.Key;
...
}
I am looking for a perticular raw SQL query using Inner Join.
I have those models:
class EzMap(models.Model):
layers = models.ManyToManyField(Shapefile, verbose_name='Layers to display', null=True, blank=True)
class Shapefile(models.Model):
filename = models.CharField(max_length=255)
class Feature(models.Model):
shapefile = models.ForeignKey(Shapefile)
I would like to make a SQL Query valid with PostgreSQL that would be like this one:
select id from "table_feature" where' shapefile_ezmap_id = 1 ;
but I dont know how to use the INNER JOIN to filter features where the shapefile they belongs to are related to a particular ezmap object
Something like this:
try:
id = Feature.objects.get(shapefile__ezmap__id=1).id
except Feature.DoesNotExist:
id = 0 # or some other action when no result is found
You will need to use filter (instead of get) if you want to deal with multiple Feature results.
I'm struggling with following scenario: Let's assume I have the two entities Classroom and Member, mapped with many-to-many. Classroom has the collection Members, containing the entities Member.
I would like to get the classrooms which do have members of a certain count. That would result in something like:
FROM Classroom cr WHERE cr.Members.size < 10
Now I have a Type on Classroom. I'd like to filter first on Type, then the size. This won't work:
FROM Classroom cr WHERE cr.Members.size < 10 AND cr.Members.Type = 1
Results in: illegal attempt to dereference collection
How could I write such a query?
I would imagine you need to do a join
from Classroom as cr left join cr.Members as m
where cr.Members.size < 10 and m.Type = 1