So I'm building a Restaurant Review site for my community. I need to
extract data from the following tables: RESTAURANT, CUISINE, CITY,
PRICE and RATING (customer ratings).
The query should return all restuarants of a selected CUISINE_ID and
return the RESTAURANT_NAME, CUSINE_NAME, CUTY_NAME, PRICE_CODE and it
should average all the reviews RATING_CODE and return a calculated
value. I'm fine with returning all the data except the average
rating.
I've only been working with LINQ to Entities 2 days and LINQ for about
3 weeks, so I'm really a newbie; I'm waiting for my LINQ book to be
delivered from Amazon.com. Your help guidance be appreciated!
It should end up looking something like this:
var avgForMatches =
(from r in context.Restaurants
where r.Cuisines.Any(c => c.CuisineName == cuisineName)
where r.Prices.Any(p => p.PriceCode == priceCode)
//... same pattern for other searches.
select r.RatingCode)
.Average();
Read about aggregate methods (including average) within the 101 linq samples - http://msdn.microsoft.com/en-us/vcsharp/aa336747
Related
I am trying to get a list from the database containing two or more lists inside that list.(using .net core, entity framework).Assume I have two table call header and details table.
Header Table
Detail Table
And I want the result like this:
{
"data":[
{
"Country":"Singapore",
"Hospital_List":[
{
"Hospital_Name":"SG Host A"
},
{
"Hospital_Name":"SG Host A"
}
]
},
{
}
]
}
I only know two ways to get the result like this,First Way, select Country list data with blank Hospital list as List,then for loop that list to select related Hospital list from db again.
And Second Way,select Country list data with blank Hospital list as IQueryable List,and then select related Hospital list via jointing with Hospital Table.So my question is
Which way should i used to get higher performance? And Is any other way?
Please remember there has a lot of field and data in my real table.
For loop give give you the lowest perfomance, because you will create SQL query for each iteration. Instead of this, try following solution:
from hospital in hospitals
group hospital by hospital.CID into gh
join country in countries
on gh.FirstOrDefault().CID equals country.CID
select new
{
Country = country.Country,
Hospital_List = from h in gh select h
}
EDITED:
And if your model created right you can use this code:
from hospital in hospitals
join country in countries
on hospital.Country equals country
group hospital by hospital.CID into gh
select new
{
Country = from h in gh select h.Country.Country,
Hospital_List = from h in gh select h
}
I'm currently working on a movie recommendation query that should return the movies with the most "recommendation impact" using the following cypher query:
match (m:Movie)
with m, size((m)<-[:LIKED]-(:User)-[:LIKED]->(:Movie)) as score
order by score desc
limit 10
return m.title, score
After reading the graphdb (neo4j) e-book my assumption was that this whould be an easy query for neo4j but the execution time took 32737 ms which is not what I was expecting. Does any one have experience with these kind of queries and has any suggestions to improve performance? Or should this query perform well and do I need to do some neo4j / java configuration tuning?
The profile of the query:
The result:
Maybe this is something you can pre-calculate.
Your score is related to the number of movies liked by each user. Why not calculate and store the number of movies liked by each user (assuming a user can only like a movie once, not multiple times)?
Note that this only makes sense if you only care about the number of movies liked by each user, and are okay with adding those up, even if they represent multiple likes of the same movie across many users.
MATCH (u:User)
SET u.likedCount = SIZE((u)-[:LIKED]->(:Movie))
You will need to update this every time the user likes (or unlikes) another movie.
When this is pre-populated for all users, your scoring query now becomes:
MATCH (m:Movie)
WITH m
MATCH (m)<-[:LIKED]-(u:User)
WITH m, SUM(u.likedCount) as score
ORDER BY score desc
LIMIT 10
RETURN m.title, score
EDIT
This of course includes the likes from each user of the movie in question. If you really need to account for this, you'll need to adjust your with to:
WITH m, SUM(u.likedCount) - count(u) as score
If you only want to count distinct movies liked by users in your scoring, then you can't pre-calculate and have to use something like stdob--'s answer.
Try this query:
MATCH (M:Movie)<-[:LIKED]-(:User)-[:LIKED]->(R:Movie)
WITH M,
size( collect(distinct R) ) as score
RETURN M.title as title,
score
ORDER BY score DESC LIMIT 10
As an option:
MATCH (M:Movie)<-[:LIKED]-(:User)-[:LIKED]->(R:Movie)
RETURN M.title as title,
count(R) as score
ORDER BY score DESC LIMIT 10
i have data table like :
country
China
India
Thailand
India
china
china
Thailand
Hong kong
India
can get my output as shown below using LINQ
Country Count
India 3
China 2
Thailand 2
Hong kong 1
As Ben Allred pointed out, what you're likely looking for is the LINQ GroupBymethod.
Using query syntax, it may look something like this:
var query = from tuple in table
group tuple by tuple.Country into g
select new { Country = g.Key, Count = g.Count() };
query now contains an IEnumerable collection of anonymous objects which have as members the string Country and the integer Count representing the number of occurrences of that country in the table.
You can now of course iterate over these objects as such:
foreach (var item in query)
{
Console.WriteLine("Country : {0} - Count : {1}", item.Country, item.Count);
}
For more examples, I strongly suggest the 101 LINQ Samples
It's also worth pointing out if you haven't used LINQ before that the processing is deferred, meaning that the iteration over the query object doesn't occur until you try to access any of its items, for example, in the foreach statement. If the collection or reading from table is expensive and you intend to use the results of the query more than once, you can call ToList() on query to return a more tangible, concrete collection.
When I use the following Linq query in LinqPad I get 25 results returned:
var result = (from l in LandlordPreferences
where l.Name == "Wants Student" && l.IsSelected == true
join t in Tenants on l.IsSelected equals t.IsStudent
select new { Tenant = t});
result.Dump();
When I add .Distinct() to the end I only get 5 results returned, so, I'm guessing I'm getting 5 instances of each result when the above is used.
I'm new to Linq, so I'm wondering if this is because of a poorly built query? Or is this the way Linq always behaves? Surely not - if I returned 500 rows with .Distinct(), does that mean without it there's 2,500 returned? Would this compromise performance?
It's a poorly built query.
You are joining LandlordPreferences with Tenants on a boolean value instead of a foreign key.
So, most likely, you have 5 selected land lords and 5 tenants that are students. Each student will be returned for each land lord: 5 x 5 = 25. This is a cartesian product and has nothing to do with LINQ. A similar query in SQL would behave the same.
If you would add the land lord to your result (select new { Tenant = t, Landlord = l }), you would see that no two results are actually the same.
If you can't fix the query somehow, Distinct is your only option.
I have a database with customers orders.
I want to use Linq (to EF) to query the db to bring back the last(most recent) 3,4...n orders for every customer.
Note:
Customer 1 may have just made 12 orders in the last hr; but customer 2 may not have made any since last week.
I cant for the life of me work out how to write query in linq (lambda expressions) to get the data set back.
Any good ideas?
Edit:
Customers and orders is a simplification. The table I am querying is actually a record of outbound messages to various web services. It just seemed easer to describe as customers and orders. The relationship is the same.
I am building a task that checks the last n messages for each web service to see if there were any failures. We are wanting a semi real time Health status of the webservices.
#CoreySunwold
My table Looks a bit like this:
MessageID, WebserviceID, SentTime, Status, Message, Error,
Or from a customer/order context if it makes it easer:
OrderID, CustomerID, StatusChangedDate, Status, WidgetName, Comments
Edit 2:
I eventually worked out something
(Hat tip to #StephenChung who basically came up with the exact same, but in classic linq)
var q = myTable.Where(d => d.EndTime > DateTime.Now.AddDays(-1))
.GroupBy(g => g.ConfigID)
.Select(g =>new
{
ConfigID = g.Key,
Data = g.OrderByDescending(d => d.EndTime)
.Take(3).Select(s => new
{
s.Status,
s.SentTime
})
}).ToList();
It does take a while to execute. So I am not sure if this is the most efficient expression.
This should give the last 3 orders of each customer (if having orders at all):
from o in db.Orders
group o by o.CustomerID into g
select new {
CustomerID=g.Key,
LastOrders=g.OrderByDescending(o => o.TimeEntered).Take(3).ToList()
}
However, I suspect this will force the database to return the entire Orders table before picking out the last 3 for each customer. Check the SQL generated.
If you need to optimize, you'll have to manually construct a SQL to only return up to the last 3, then make it into a view.
You can use SelectMany for this purpose:
customers.SelectMany(x=>x.orders.OrderByDescending(y=>y.Date).Take(n)).ToList();
How about this? I know it'll work with regular collections but don't know about EF.
yourCollection.OrderByDescending(item=>item.Date).Take(n);
var ordersByCustomer =
db.Customers.Select(c=>c.Orders.OrderByDescending(o=>o.OrderID).Take(n));
This will return the orders grouped by customer.
var orders = orders.Where(x => x.CustomerID == 1).OrderByDescending(x=>x.Date).Take(4);
This will take last 4 orders. Specific query depends on your table / entity structure.
Btw: You can take x as a order. So you can read it like: Get orders where order.CustomerID is equal to 1, OrderThem by order.Date and take first 4 'rows'.
Somebody might correct me here, but i think doing this is linq with a single query is probably very difficult if not impossible. I would use a store procedure and something like this
select
*
,RANK() OVER (PARTITION BY c.id ORDER BY o.order_time DESC) AS 'RANK'
from
customers c
inner join
order o
on
o.cust_id = c.id
where
RANK < 10 -- this is "n"
I've not used this syntax for a while so it might not be quite right, but if i understand the question then i think this is the best approach.