I am trying to retrieve a collection of data by an order like this:
#all_data= Data.find(#data_ids)
#data_ids - have the ids of the data to be retrieved. For instance:
Data
ID Name
1 A
2 B
3 C
4 D
If the #data_ids are like [3,2,4] I want to retrieve the data in that order, sou it would be C, B, D... The thing is it always retrieve the data in B, C, D order. Is that possible to do? To ignore that order and to retrieve it by the given params order?
The thing is,
I have two tables, table A and table Data.
Table A:
Relation, Data_ID, ORDER
1; 1; 2;
1; 2; 1;
2; 3; 3;
So what I want to do is to retrieve data_id by order from the relation one I will have (2,1), and I am doing it, but when I find (2,1) I receive 1,2.
In MySQL there is a function that gives you the power to override custom ordering - take a look at "Sort by specific ids in ActiveRecord".
If you are using Postgres, you may consider this approach (credit to
Omar Qureshi for his answer in "ActiveRecord.find(array_of_ids), preserving order":
unsorted = Model.find(arr)
sorted = arr.inject([]){|res, val| res << unsorted.detect {|u| u.id == val}}
Just fetch records from database, and sort them in memory. This works when what you wanna fetch is relatively a small amount.
Data.find(#data_ids).sort_by!{|data| #data_ids.index(data.id)}
P.S. the reason why you always get the data in id's order is that primary keys are always indexed, and the database traverses indexes in some order (asc or desc).
Related
I'm trying to read data from related tables which are x levels deep and which have relationships specified, i.e.:
table A
table B
table C
table ABC
Table ABC has relationships ABC.a = A, ABC.b = B and ABC.c = C,
i.e. foreign keys ABC.aid = A.id, ABC.bid = B.id and ABC.cid = C.id.
aid, bid and cid in ABC are set unique using UniqueContstraint
relationship is using lazy="joined"
When I do select(ABC) I'm able to get all values from ABC and also from related tables, i.e.:
{ABC.a: {A}, ABC.b: {B}, ABC.c: {C}}
I have also table D which has a relationship to ABC (D.abcid = ABC.id) and I struggle to construct a correct select statement which would give me all data also from A, B and C. Actually I'm not sure if this should work or I missed / do not understand something in the documentation as I have tried various loading strategies, specified join_depth for D and ABC tables, etc. No matter what I'm getting:
sqlalchemy.exc.InvalidRequestError: The unique() method must be invoked on this Result, as it contains results that include joined eager loads against collections
I would like to get the data the same way as for 1st level relationship, i.e.:
{D.abc : {ABC.a: {A}, ABC.b: {B}, ABC.c: {C}}}
Is it possible or do I have to change the select query completely and just create multiple joins and manually pick all the values I need?
I'm able to get correct records from the database when I just take the generated select statement and use it directly in a DB shell (MariaDB) so I assume that the only issue is my lack of understanding of how SQL handles/presents these records internally.
The issue was using uselist=True in one of the models, all relationships are working perfectly down to the lowest level now.
I want to be able to limit the activerecord objects to 20 being returned, then perform a where() that returns a subset of the limited objects which I currently know only 10 will fulfil the second columns criteria.
e.g. of ideal behaviour:
o = Object.limit(20)
o.where(column: criteria).count
=> 10
But instead, activerecord still looks for 20 objects that fulfil the where() criteria, but looks outside of the original 20 objects that the limit() would have returned on its own.
How can I get the desired response?
One way to decrease the search space is to use a nested query. You should search the first N records rather than all records which match a specific condition. In SQL this would be done like this:
select * from (select * from table order by ORDERING_FIELD limit 20) where column = value;
The query above will only search for the condition in 20 rows from the table. Notice how I have added a ORDERING_FIELD, this is required because each query could give you a different order each time you run it.
To do something similar in Rails, you could try the following:
Object.where(id: Object.order(:id).limit(20).select(:id)).where(column: criteria)
This will execute a query similar to the following:
SELECT [objects].* FROM [objects] WHERE [objects].[id] IN (SELECT TOP (20) [objects].[id] FROM [objects] ORDER BY [objects].id ASC) AND [objects].[column] = criteria
I have an Esper query that returns multiple rows, but I'd like to instead get one row, where that row has a list (or concatenated string) of all of the values from the (corresponding columns of the) matching rows that my current query returns.
For example:
SELECT Name, avg(latency) as avgLatency
FROM MyStream.win:time(5 min)
GROUP BY Name
HAVING avgLatency / 1000 > 60
OUTPUT last every 5 min
Returns:
Name avgLatency
---- ----------
A 65
B 70
C 75
What I'd really like:
Name
----
{A, B, C}
Is this possible to do via the query itself? I tried to make this work using subqueries, but I'm not working with multiple streams. I can't find any aggregation functions or enumeration functions in the Esper documentation that fits what I'm trying to do either.
Thanks to anybody that has any insight or direction for me here.
EDIT:
If this can't be done via the query, I'm open to changing the subscriber, or anything else, if necessary.
You can have a subscriber or listener do the concat. There is a "Multi-Row Delivery" for subscribers. Or use a table like below.
// create table to hold aggregation result
create table LatencyTable(name string primary key, avgLatency avg(double));
// update aggregations in table from events coming in
into LatencyTable select name, avg(latency) as avgLatency from MyStream#time(5 min) group by name;
// do a select with the "aggregate" enumeration method
select (select * from LatencyTable where avgLatency > x).aggregate(....) from pattern[every timer:interval(5 min)]
I want to get the number of selected rows as well as the selected data. At the present I have to use two sql statements:
one is
select * from XXX where XXX;
the other is
select count(*) from XXX where XXX;
Can it be realised with a single sql string?
I've checked the source code of sqlite3, and I found the function of sqlite3_changes(). But the function is only useful when the database is changed (after insert, delete or update).
Can anyone help me with this problem? Thank you very much!
SQL can't mix single-row (counting) and multi-row results (selecting data from your tables). This is a common problem with returning huge amounts of data. Here are some tips how to handle this:
Read the first N rows and tell the user "more than N rows available". Not very precise but often good enough. If you keep the cursor open, you can fetch more data when the user hits the bottom of the view (Google Reader does this)
Instead of selecting the data directly, first copy it into a temporary table. The INSERT statement will return the number of rows copied. Later, you can use the data in the temporary table to display the data. You can add a "row number" to this temporary table to make paging more simple.
Fetch the data in a background thread. This allows the user to use your application while the data grid or table fills with more data.
try this way
select (select count() from XXX) as count, *
from XXX;
select (select COUNT(0)
from xxx t1
where t1.b <= t2.b
) as 'Row Number', b from xxx t2 ORDER BY b;
just try this.
You could combine them into a single statement:
select count(*), * from XXX where XXX
or
select count(*) as MYCOUNT, * from XXX where XXX
To get the number of unique titles, you need to pass the DISTINCT clause to the COUNT function as the following statement:
SELECT
COUNT(DISTINCT column_name)
FROM
'table_name';
Source: http://www.sqlitetutorial.net/sqlite-count-function/
For those who are still looking for another method, the more elegant one I found to get the total of row was to use a CTE.
this ensure that the count is only calculated once :
WITH cnt(total) as (SELECT COUNT(*) from xxx) select * from xxx,cnt
the only drawback is if a WHERE clause is needed, it should be applied in both main query and CTE query.
In the first comment, Alttag said that there is no issue to run 2 queries. I don't agree with that unless both are part of a unique transaction. If not, the source table can be altered between the 2 queries by any INSERT or DELETE from another thread/process. In such case, the count value might be wrong.
Once you already have the select * from XXX results, you can just find the array length in your program right?
If you use sqlite3_get_table instead of prepare/step/finalize you will get all the results at once in an array ("result table"), including the numbers and names of columns, and the number of rows. Then you should free the result with sqlite3_free_table
int rows_count = 0;
while (sqlite3_step(stmt) == SQLITE_ROW)
{
rows_count++;
}
// The rows_count is available for use
sqlite3_reset(stmt); // reset the stmt for use it again
while (sqlite3_step(stmt) == SQLITE_ROW)
{
// your code in the query result
}
Let say a book model HABTM categories, for an example book A has categories "CA" & "CB". How can i retrieve book A if I query using "CA" & "CB" only. I know about the .where("category_id in (1,2)") but it uses OR operation. I need something like AND operation.
Edited
And also able to get books from category CA only. And how to include query criteria such as .where("book.p_year = 2012")
ca = Category.find_by_name('CA')
cb = Category.find_by_name('CB')
Book.where(:id => (ca.book_ids & cb.book_ids)) # & returns elements common to both arrays.
Otherwise you'd need to abuse the join table directly in SQL, group the results by book_id, count them, and only return rows where the count is at least equal to the number of categories... something like this (but I'm sure it's wrong so double check the syntax if you go this route. Also not sure it would be any faster than the above):
SELECT book_id, count(*) as c from books_categories where category_id IN (1,2) group by book_id having count(*) >= 2;