We have the following SQL query:
SELECT ID, (SELECT TOP(1) COMPANYNAME FROM dbo.table1) AS COMPANYNAME FROM dbo.table2
How can we achieve the same query in LINQ, in a single attempt?
I tried the following code
var qry=from o in db.table2.tolist()
Select new {
o.ID,
COMPANYNAME=(db.table1.firstordefault())
};
But unfortunately an exception occurred i.e all threads need to be run etc...
Please help me out,
I googled but could not find a positive relevant result.
var query = db.table2
.Select(s => new
{
s.ID,
db.table1.Select(n => n.COMPANYNAME).FirstOrDefault()
})
.ToList()
Related
I have a linq query which gave me the warning but it still works. I want to get rid of the warning.
uses First/FirstOrDefault/Last/LastOrDefault operation without OrderBy and filter which may lead to unpredictable results.
The linq query is
var list = (from u in _db.user
join r in _db.resource on u.userId equals r.userId
join t in _db.team on u.bossId equals t.bossId
where r.pid == pid
select new MyDto
{
pid = pid,
userId = u.userId,
teamId = t.teamId,
name = t.name
}).GroupBy(d => d.userId).Select(x => x.First()).OrderBy(y => y.userId).ToList();
I use EntityFramework Core 2.1
UPDATE:
I changed the code by the comments.
var list = (from u in _db.user
join r in _db.resource on u.userId equals r.userId
join t in _db.team on u.bossId equals t.bossId
where r.pid == pid
select new MyDto
{
pid = pid,
userId = u.userId,
teamId = t.teamId,
name = t.name
})
.GroupBy(d => d.userId)
.Select(x => x.OrderBy(y => y.userId)
.First())
.ToList();
Then there is a different warning.
The LINQ expression 'GroupBy([user].userId, new MyDto() {pid =
Convert(_8_locals1_pid_2, Int16), userId = [user].UserId, .....) could
not be translated and will be evaluated locally.
We have this expression
.Select(x => x.First())
Which record will be first for that expression? There's no way to know, because at this point the OrderBy() clause which follows hasn't processed yet. You could get different results each time you run the same query on the same data, depending on what order the records were returned from the database. The results are not predictable, exactly as the error message said.
But surely the database will return them in the same order each time? No, you can't assume that. The order of results in an SQL query is not defined unless there is an ORDER BY clause with the query. Most of the time you'll get primary key ordering (which does not have to match insert order!), but there are lots of things that can change this: matching a different index, JOIN to a table with a different order or different index, parallel execution with another query on the same table + round robin index walking, and much more.
To fix this, you must call OrderBy() before you can call First().
Looking a little deeper, this is not even part of the SQL. This work is happening on your client. That's not good, because any indexes on the table are no longer available. It should be possible to do all this work on the database server, but selecting the first record of a group may mean you need a lateral join/APPLY or row_number() windowing function, which are hard to reproduce with EF. To completely remove all warnings, you may have to write a raw SQL statement:
select userId, teamId, name, pid
from (
select u.userId, t.teamId, t.name, r.pid, row_number() over (order by u.userId) rn
from User u
inner join resource r on r.userId = u.userId
inner join team t on t.bossId = u.bossId
where r.pid = #pid
) d
where d.rn = 1
Looking around, it is possible to use row_number() in EF, but at this point I personally find the SQL much easier to work with. My view is ORMs don't help for these more complicated queries, because you still have to know the SQL you want, and you also have to know the intricacies of the ORM in order to build it. In other words, the tool that was supposed to make your job easier made it harder instead.
Could somebody assist me in converting a sql query into LINQ ? I well understand SQL queries, but I am a novice in Linq. Thank you so much for help me.
SELECT
subConsulta."NitIps",
subConsulta."NumFactura",
COUNT(*)
FROM
(SELECT
DISTINCT acf."NitIps",
acf."NumFactura",
acf."TipoSoporte"
FROM
"t_ArchivoCentralFacturacion" AS acf
inner join "t_TRCompartaTiposDocumentalesAC" AS ctd
on
acf."TipoSoporte"= ctd."Id"
GROUP BY
acf."NitIps",
acf."NumFactura",
acf."TipoSoporte")as subConsulta
GROUP BY
subConsulta."NitIps",
subConsulta."NumFactura"
ORDER BY
subConsulta."NitIps",
subConsulta."NumFactura"
If you map your tables to entities it looks like follow:
var first = from archivoCentralFacturacion in ArchivoCentralFacturacions
group archivoCentralFacturacion by new {
c.NitIps,
c.NumFactura,
c.TipoSoporte
} into subConsulta
select subConsulta;
var result = (from f in first
group f by new {
f.NitIps,
f.NumFactura
} into r
select new {
NitIps = r.NitIps,
NumFactura = r.NumFactura,
ResultCount = r.Count()
}).OrderBy(x => x.NitIps).ThenBy(x => x.NumFactura);
I am attempting to write the following SQL as a linq query.
SELECT grp.OrganisationId,
grp.OrderCount,
organisations.Name
FROM (select OrganisationId,
count(*) as OrderCount
from orders
where 1 = 1
group by OrganisationId) grp
LEFT OUTER JOIN organisations on grp.OrganisationId = organisations.OrganisationId
WHERE 1 = 1
The where clauses are simplified for the benefit of the example.
I need to do this without the use of navigational properties.
This is my attempt:
var organisationQuery = ClientDBContext.Organisations.Where(x => true);
var orderGrouped = from order in ClientDBContext.Orders.Where(x => true)
group order by order.OrganisationId into grouping
select new { Id = grouping.Key.Value, OrderCount = grouping.Count() };
var orders = from og in orderGrouped
join org in organisationQuery on og.Id equals org.Id
select(x => new OrganisationOrdersReportPoco()
{
OrganisationNameThenCode = org.Name,
TotalOrders = og.OrderCount
});
But I am getting an error of...
Type inference failed in the call to 'Join'
From previous threads, I believe this is because I have "lost the join with order" (but I don't understand why that matters when I am creating a new recordset of Organisation, Count).
Thanks!
I understand you may believe navigation properties are the solution here, but if possible, please can we keep the discussion to the join off of the group by as this is the question I am trying to resolve.
You are mixing lambda and LINQ expressions. Change select to:
select new OrganisationOrdersReportPoco()
{
OrganisationNameThenCode = org.Name,
TotalOrders = og.OrderCount
};
If i understood your model correctly you could try this instead:
var orders = ClientDBContext.Organisations.Select(org => new OrganisationOrdersReportPoco
{
OrganisationNameThenCode = org.Name,
TotalOrders = org.Orders.Count()
}).ToList();
I am getting the following error on the word "join" in the code below.
The type of one of the expressions in the join clause is incorrect.
Type inference failed in the call to 'Join'.
var organisationQuery = ClientDBContext.Organisations.Where(x => true);
var orderGrouped = from order in ClientDBContext.Orders.Where(x => true)
group order by order.OrganisationId into grouping
select new { Id = grouping.Key.Value, OrderCount = grouping.Count() };
var orders = from og in orderGrouped
join org in organisationQuery on og.Id equals org.Id
select(x => new OrganisationOrdersReportPoco()
{
OrganisationNameThenCode = org.Name,
TotalOrders = og.OrderCount
});
I don't see a problem with the join clause? Can anyone please advise?
Edit:
This is the piece of SQL I am attempting to write as LINQ.
SELECT grp.OrganisationId,
grp.OrderCount,
organisations.Name
FROM (select OrganisationId,
count(*) as OrderCount
from orders where 1 = 1 group by OrganisationId) grp
LEFT OUTER JOIN organisations on grp.OrganisationId = organisations.OrganisationId
WHERE 1 = 1
I have complicated where clauses on both orders and organisations... simplified for this example.
You are selecting into an anonymous type in the first query:
var orderGrouped = ..
select new { Id = grouping.Key.Value, OrderCount = grouping.Count() };
This 'breaks' the connection with order.
The join looks like it should work for Linq-to-Objects but it can't be converted into SQL.
You'll have to eliminate the anonymous type and somehow make a more direct connection.
I wonder why you don't simply go from Organisations? With a proper mapping using nav-properties it should look like:
from org in ClientDBContext.Organisations
select(x => new OrganisationOrdersReportPoco()
{
OrganisationNameThenCode = org.Name,
TotalOrders = org.Orders.Count
};
using the Id properties should be a little more involved but follow the same pattern.
(Credit to Giorgi Nakeuri)
I was confusing LAMBDA with LINQ expressions.
Replacing my select with this solved it.
select new OrganisationOrdersReportPoco()
{
OrganisationNameThenCode = org.Name,
TotalOrders = og.OrderCount
};
I have the query:
SELECT TOP 50 CONVERT(date, o.OrderDate)as OrderDate,ISNULL(rd.SerialNumbers,'') as SerialNumbers,CAST(o.SourceOrderID as varchar(50)) as SourceOrderNumber
From Orders o
Query is edited for question.
var lq= (
from o in db.Orders
select new {,o.Name, o.Company, o.Address, o.Address2, o.City, o.State, o.Country, o.Email, o.Zip, o.Phone, o.ShipName, o.ShipCompany, o.ShipAddress, o.ShipAddress2, o.ShipCity, o.ShipCountry, o.ShipState, o.ShipPhone, o.ShipZip, o.OrderNumber, o.ShippingTotal }
).ToList();
I can make simple joins and select in LINQ but not getting idea how to get selects like one mentioned in query.
I am using EF
Given there is no way to actually perform a string to datetime conversion as part of the DB query using LINQ, it's recommended you use AsEnumerable to switch the context of the list to run the actual conversion in memory
orders.Take(50)
.AsEnumerable()
.Select(x => new {
OrderDate = x.OrderDate.Date,
SerialNumbers = o.SerialNumbers ?? "",
SourceOrderNumber = o.SourceOrderID.ToString()
});
LINQ2SQL is actually bright enough to handle the parse in the code:
var q = from c in Customers
where c.PhoneNumber == "9075556658"
select new
{
SSN = c.SSN,
DOB = DateTime.Parse(c.BirthDate)
};
q.Dump();
I just run that in LinqPad on my own database, and it worked fine.
NOTE: This was tested with Linq2SQL, not Entity Framework, which has a history of not be able to do things Linq2SQL has been doing for years.