I have user with name, location, created_at as important fields in table.
I want to retrieve for each user the latest location,i.e, I want something like this:
username location created_at
abc New York 2012-08-18 16:18:57
xyz Mexico city 2012-08-18 16:18:57
abc Atlanta 2012-08-11 16:18:57
only input is UId(1,2) array of userids.please help me to accomplish this.I just want to know how to write query using active record query interface.
Generally, this should be a standard way to solve this kind of problems:
SELECT l1.user, l1.location
FROM locations l1
LEFT JOIN locations l2 ON l1.user = l2.user AND l2.created_at > l1.created_at
WHERE l2.id IS NULL
The idea is to join the table with itself, and find those rows which don't have any row with the same user and greater created_at.
Of course, you should have (user, created_at) index on your table.
Now you should see how would that be represented in AR interface.
When
u_id
is the array of user ids, then
u_id.map{|i| User.find(i).location}
should be an array of the users locations.
You can Use
User.where(:uid => [1,2,3]).maximum('location')
which will create something like
SELECT MAX(`users`.`location`) AS max_id FROM `users` WHERE `users`.`id` IN (1, 2,3)
Related
I'm relatively new to Oracle SQL and have run into an issue where I'm trying to filter a report to only return records logged by a specific list of user names.
They are currently stored in the system in fields user.first_name and user.surname and I've created the following CAST field in the coding to join the two together:
CAST(USER.FIRST_NAME||' '||USER.SURNAME as VARCHAR (25)) as CUSTOMER
What I want to do now though is restrict it so that my query will only return records where the customer is in a pre-determined list that I can hard core into the SQL.
eg I only want to see records for :
Joe Bloggs,
John Doe,
A Nother
How do I do this in Oracle SQL?
Thanks
One option (as you said you hardcoded it) is
select *
from your_table
where customer in ('Joe Bloggs', 'John Doe', 'A Nother');
A better one is to store those customers into a separate table and join it with your_table:
insert into separate_table (name) values ('Joe Bloggs'); -- do the same for the rest
select *
from your_table y join separate_table s on s.name = y.first_name ||' '||i.surname;
Even better, use their IDs (because, there could be two John Doe persons; which one will you take)?
insert into separate_table (id) values (1123); -- this is Joe Bloggs
select *
from your_table y join separate_table s on s.id = y.id;
TablePatient.Patient_ID(PK)
TableProviders.Encounter (joins to PK)
TableProviders.Provider_Type
TableProviders.Provider_ID
TableNames.Full_Name
TableNames.Provider_ID (joins to Table Names)
I want a query that will give, for all the Patient_IDs, the Full_Name of the provider for every Provider ID.
There are about 30 provider_types.
I have made this already using a left join a ton of left joins. It takes a long time to run and I am thinking there is a trick I am missing.
Any help?
Ok, my previous answer didn't match at all what you meant. You want to pivot the table to have on each line one Patient_ID with every Full_name for every provider_type. I assume that each patient has only one provider for one type and not more ; if more, you will have more than one row for each patient, and anyway I don't think it's really possible.
Here is my solution with pivot. The first part is to make it more understandable, so I create a table named TABLE_PATIENT in a subquery.
WITH TABLE_PATIENT AS
(
SELECT TablePatient.Patient_ID,
TableProviders.Provider_Type,
TableNames.Full_Name
FROM TablePatient LEFT JOIN
TableProviders on TablePatient.Patient_ID = TableProviders.Encounter
LEFT JOIN
TableNames on TableNames.Provider_ID = TableProviders.Provider_ID
group by TablePatient.Patient_ID,
TableProviders.Provider_Type,
TableNames.Full_Name
)
SELECT *
FROM TABLE_PATIENT
PIVOT
(
min(Full_name)
for Provider_type in ([type1], [type2],[type3])
) AS PVT
So TABLE_PATIENT just has many rows for each patient, with one provider each row, and the pivot puts everything on a single row. Tell me if something doesn't work.
You need to write every type you want in the [type1],[type2] etc. Just put them inside [], no other character needed as ' or anything else.
If you put only some types, then the query will not show providers of other types.
Tell me if something doesn't work.
If I understand what you mean, you just want to group the answer by Patient Id and then Provider ID. A full name is unique on a provider id right ?
This should be something like
SELECT TablePatient.Patient_ID,
TableProviders.Provider_ID,
TableNames.Full_Name
FROM TablePatient LEFT JOIN
TableProviders on TablePatient.Patient_ID = TableProviders.Encounter
LEFT JOIN
TableNames on TableNames.Provider_ID = TablerProviders.Provider_ID
group by TablePatient.Patient_ID,
TableProviders.Provider_ID,
TableNames.Full_Name
You can either group by TableNames.Full_Name or select First(TableNames.Full_Name) for example if indeed a full name is unique to a provider ID.
Note : I used the SQL server Syntax, there can be différences with Oracle ..
Suppose, I have a student table with some fields in impala. Imagine there is a field called total_mark and I should find the student details with maximum mark from each branch.
My table is like this :-
In this table I have to get the details of student with maximum marks from each department.
My query will be like this :-
select id,max(total_marks) from student_details group by department;
But using this query I can get only the id and total_marks. Provided there can be students with same name,age I can't group with fields like age,name .
So how should I query the table to get all the details of top student from each department ??
Thanks in advance.
You can make use of the JOIN concept
select stu.*
from student_details stu
join
( select department,max(total_marks) as max
from student_details
group by department
) rank
on stu.department=rank.department and stu.total_marks=rank.max;
I have the following tables:
Table1:
user_name Url
Rahul www.cric.info.com
ranbir www.rogby.com
sahil www.google.com
banit www.yahoo.com
Table2:
Keyword category
cric sports
footbal sports
google search
I want to search Table1 by matching the keyword in Table2. I can perform the same using case statement and the query works but it is not the right approach because each time I have to add the case statement when I will add new search keyword.
select user_name from table1
case when url like '%cric%' then sports
else 'undefined'
end as category
from table1;
Thanks find the soluntions for this approach. FIrst we need to do the Join and after that we need to filter the record.
select user_name,url,Keyword,catagory from(select table1.user_name,table1.url ,table2.keyword,table2.catagory from table1 left outer join table2)a where a.url like (concat('%',a.phrase,'%')
Not sure about more current versions, but I've run into a similar problem... the primary issue is that Hive only supports equi-join statements... when you apply logic to either side of the join, it has difficulty translating into a Map Reduce function.
The alternative method, if you have a reliably structured field, is that you can create a matching key from the larger field. For example, if you know that you're looking for your keyword to exist in the second position of a dot-delimited URI, you could do something like:
select
Uri
, split(Uri, "\\.")[1] as matchKey
from
Table1
join Table2 on Table2.keyword = Table1.matchKey
;
I have a Contact model which has many Notes. On one page of my app, I show several attributes of a table of contacts (name, email, latest note created_at).
For the note column, I'm trying to write a joins statement that grabs all contacts along with just their latest note (or even just the created_at of it
What I've come up with is incorrect as it limits and orders the contacts, not their notes:
current_user.contacts.joins(:notes).limit(1).order('created_at DESC')
If you just want the created_at value for the most recent note for each contact, you can first create a query to find the max value and then join with that query:
max_times = Note.group(:contact_id).select("contact_id, MAX(created_at) AS note_created_at").to_sql
current_user.contacts.select("contacts.*, note_created_at").joins("LEFT JOIN (#{max_times}) max_times ON contacts.id = max_times.contact_id")
If you want to work with the Note object for the most recent notes, one option would be to select the notes and group them by the contact_id. Then you can read them out of the hash as you work with each Contact.
max_times = Note.group(:contact_id).select("contact_id, MAX(created_at) AS note_created_at").to_sql
max_notes = Note.select("DISTINCT ON (notes.contact_id) notes.*").joins("INNER JOIN (#{max_times}) max_times ON notes.contact_id = max_times.contact_id AND notes.created_at = note_created_at").where(contact_id: current_user.contact_ids)
max_notes.group_by(&:contact_id)
This uses DISTINCT ON to drop dups in case two notes have exactly the same contact_id and created_at values. If you aren't using PostgreSQL you'll need another way to deal with dups.