I have multiple Link queries loading to a IEnumerable list
var queries = new List<IEnumerable<Customers>>();
queries.Add(from c in context.Customers where c.region = 'NE' select c);
queries.Add(from c in context.Customers where c.region = 'SW' select c);
//want count of customers in those two regions
var result = queries.Sum(q => Count());
However it is returning a count of the queries (2), not count of the Customers.
How can execute the List of IEnumerable queries?
The mistake is at the last line:
q => Count()
Instead of actually counting the elements in a sequence, which would be q => q.Count(), you are calling a method Count() of either a current instance or the current class.
Actually, there is no need to do two separate queries. Instead, use a single query:
var query = from c
in context.Customers
where c.region = 'NE' || c.region = 'SW'
select c;
var result = query.Count();
or, slightly shorter:
var result = context.Customers.Count(c => c.region = 'NE' || c.region = 'SW');
Related
I'm trying to convert my SQL statement to a Linq statement and I'm not sure how to add the second COUNT to it. This is my SQL statement
SELECT l.Campus_Name, Labs = COUNT(*), LabsWithSubnets = COUNT(s.Lab_Space_Id)
FROM vw_Lab_Space l
LEFT JOIN vw_Subnet s on l.Lab_Space_Id = s.Lab_Space_Id
GROUP BY l.Campus_Name
ORDER BY 1
and this is my LINQ statement so far:
from l in Vw_Lab_Space
from s in Vw_Subnet
.Where(s => s.Lab_Space_Id == l.Lab_Space_Id)
.DefaultIfEmpty() // <=- triggers the LEFT JOIN
group l by new { l.Campus_Name } into g
orderby g.Key.Campus_Name
select new {
Campus_Name = g.Key.Campus_Name,
Labs = g.Count()
}
So I have everything but the LabsWithSubnets part in there. I'm just not sure how to add that in as I can't just do an s.Lab_Space_id.Count() in the select statement.
If you need table structure and sample data please see Need help creating an OUTER JOIN to count spaces.
Using your query as a basis, you need the groups to include s so you can count when non-null (I also removed the unnecessary anonymous object around the grouping key):
from l in Vw_Lab_Space
from s in Vw_Subnet
.Where(s => s.Lab_Space_Id == l.Lab_Space_Id)
.DefaultIfEmpty() // <=- triggers the LEFT JOIN
group new { l, s } by l.Campus_Name into g
orderby g.Key
select new {
Campus_Name = g.Key,
Labs = g.Count(),
LabsWithSubnets = g.Count(ls => ls.s != null)
}
However, rather than translate the SQL, I would probably take advantage of LINQ's group join to handle the query slightly differently:
var ans = from l in Vw_Lab_Space
join s in Vw_Subnet on l.Lab_Space_Id equals s.Lab_Space_Id into sj
group new { l, sj } by ls.Campus_Name into lsjg
select new {
Campus_Name = lsjg.Key,
NumLabs = lsjg.Count(),
LabsWithSubnets = lsjg.Sum(lsj => lsj.sj.Count())
};
PS Even in your query, I would use join...from...DefaultIfEmpty rather than from...from...where but depending on your database engine, may not matter.
I have a LINQ statement that returns many columns. I need to find distinct of unique combination of two columns. What is the best way to do this.
var productAttributeQuery =
from pa in ctx.exch_productattributeSet
join pp in ctx.exch_parentproductSet
on pa.exch_ParentProductId.Id equals pp.Id
join ep in ctx.exch_exchangeproductSet
on pp.exch_parentproductId equals ep.exch_ParentProductId.Id
where pa.exch_EffBeginDate <= effectiveDateForBeginCompare
&& pa.exch_EffEndDate >= effectiveDateForEndCompare
&& pa.statuscode == StusCodeEnum.Active
where pp.exch_EffBeginDate <= effectiveDateForBeginCompare
&& pp.exch_EffEndDate >= effectiveDateForEndCompare
&& pp.statuscode == StatusCodeEnum.Active
where ep.statuscode == StatusCodeEnum.Active
select new ProductAttributeDto
{
ParentProductId = pa.exch_ParentProductId.Id,
AttributeId = pa.exch_AttributeId.Id,
AttributeValue = pa.exch_Value,
AttributeRawValue = pa.exch_RawValue
};
return productAttributeQuery.ToList();
I want to get Distinct combination of ParentProductId and AttributeId from this list
You can group by anonymous type and select keys (they will be distinct)
var query = from p in productAttributeQuery
group p by new {
p.ParentProductId,
p.AttributeId
} into g
select g.Key;
You can use same approach with you original query if you want to get distinct pairs on server side.
Another approach - project results into pairs and get distinct from them:
var query = productAttributeQuery
.Select(p => new { p.ParentProductId, p.AttributeId })
.Distinct();
I want to know how to make Linq expression that has the same effect as these SQL query
SELECT item.*, priceforitem.*
FROM
item
LEFT JOIN priceforitem
ON priceforitem.ItemID = item.ItemID
AND priceforitem.PriceID = ?PriceID
I already make it using the Method query but I don't know if it will produce the same result
db.Items
.GroupJoin(
db.PriceForItems.Where(pi => pi.PriceID == id),
i => i.ItemID,
pi => pi.ItemID,
(i, pi) => new { Item = b, Prices = pi })
.SelectMany(
a => a.Prices.DefaultIfEmpty(),
(i, pi) => new
{
ItemID = i.Item.ItemID,
Code = i.Item.Code,
Name = i.Item.Name,
PriceForItemID = pi.PriceForItemID,
Price = pi.Price
})
and then after thinking for awhile i shorten it like this
db.Items
.SelectMany(
i => db.PriceForItems.Where(
pi => pi.PriceID == id
&& pi.ItemID = i.ItemID).DefaultIfEmpty(),
(i, pi) => new
{
ItemID = i.Item.ItemID,
Code = i.Item.Code,
Name = i.Item.Name,
PriceForItemID = pi.PriceForItemID,
Price = pi.Price
})
I am new to Linq, and I don't know which is better and how to convert it to Linq query statement.
First of all your sql query. It is effectively and inner join because the where condition will filter out all rows where data from priceforitem is null. If you do want to convert same query to linq you can do it like
from i in db.Items
join p in db.PriceforItems on
i.ItemId equals p.ItemId into tempvals
from t in tempvals.DefaultIfEmpty()
where t.PriceId == id
select new{i.ItemId, ..., t.PriceId, t...., t....}
I mostly write linq queries instead of expressions where they are more readable to me. If you still want to get an expression, you can write a valid linq query and paste it into Linqpad and it will give the result as well as lambda expression of your query.
I have this code:
var query = from deal in db.Deals
where deal.EndDate >= DateTime.UtcNow
select deal;
var priceList = filters.Price.GetPriceRangeList();
foreach(var price in priceList)
{
var startPrice = price.StartPrice;
var endPrice = price.EndPrice;
var priceResult = from deal in query
where (deal.DiscountPrice >= startPrice && deal.DiscountPrice <= endPrice)
select deal;
if(priceResult.Count() != 0)
priceResults = (priceResults == null) ? priceResult : priceResult.Union(priceResults);
}
query = priceResults != null ? query.Intersect(priceResults) : Enumerable.Empty<Deal>().AsQueryable();
My query is slow when priceList has more ten values.
I use Intersect for filters.
How to optimize these queries?
One idea for optimization would be to sort query by StartPrice ascending, that way your inner query can just stop traversal once StartPrice is higher than the DiscountPrice property:
var query = from deal in db.Deals
where deal.EndDate >= DateTime.UtcNow
orderby deal.DiscountPrice ascending
select deal;
..
foreach(..)
{
var startPrice = price.StartPrice;
var endPrice = price.EndPrice;
var queryLocal = query.SkipWhile(deal => deal.DiscountPrice < startPrice);
var priceResult = queryLocal.TakeWhile(deal => deal.DiscountPrice >= startPrice
&& deal.DiscountPrice <= endPrice);
..
}
You have a few issues. The first one is that the query is executed every iteration in your foreach loop. Calling ToList or ToArray will ensure that it is only executed once
Secondly the union is costly. It will iterate priceResult for every iteration of the foreach loop
thirdly your count will also iterate priceResult. Use .Any instead if you wish to know if theres any elements. However I think you can avoid that. If I've read your code correctly I believe the below should have the same result but it does not have the above three issues
var query = (from deal in db.Deals
where deal.EndDate >= DateTime.UtcNow
orderby deal.DiscountPrice ascending
select deal).ToList();
var priceResults = (from price in filters.Price.GetPriceRangeList()
let startPrice = price.StartPrice
let endPrice = price.EndPrice
select query.SkipWhile(d => deal.DiscountPrice < startPrice)
.TakeWhile(d => deal.DiscountPrice <= endPrice)
).SelectMany(x => x);
instead of iterating for each union there's a distinct only once
The following code works, but it's not a nice code. (low performance)
I have a dictionary with value and key.
First i go trough every webcodes who exist. Then i load all participants in a list (where webcode equals the actual webcode in the foreach). After that i add the data (parameter of the webcode and a count of participants to the dictionary).
Guid compID = Guid.Parse(wID);
ChartModel webcodes = new ChartModel();
webcodes.Title = "Webcodes Statistics";
webcodes.Data = new Dictionary<string, int>();
var webcodesData = db.t_Webcode;
foreach (var w in webcodesData)
{
var wData = db.t_Participant.Where(t => t.FK_Competition == compID && t.Webcode == w.Webcode);
if (wData.Count() != 0)
webcodes.Data.Add(w.Parameter, wData.Count());
}
ViewBag.Webcodes = webcodes;
TIA
You need something along these lines:
webcodes.Data = (from w in db.t_Webcode
join p in db.t_Participant on w.Webcode equals p.Webcode
where p.FK_Competition == compID
group w by w.Parameter into g
select new { g.Key, Count = g.Count() }).ToDictionary();
I can't test it but that is the type of query you need.
This will assume that you have relationships defined in your database and that your LINQ to SQL datacontext are aware of them. If not, you will need to join manually on t_Participants from tWebcode.
This should execute in 1 single SQL query, instead of 1 query per row in tWebcode.
var webcodesAndNoOfParticipants =
from webcode in db.tWebcode
// Define number of participants for this webcode
let numberOfParticipants = webcode.t_Participants.Count(participant => participant.FK_Competition == compID)
where numberOfParticipants > 0
select new {
WebcodeParameter = webcode.Parameter,
NoOfParticipants = numberOfParticipants
};
webcodes.Data = webcodesAndNoOfParticipants.ToDictionary(x => x.WebcodeParameter, x => x.NoOfParticipants);