Hello this is my linq query,
var RoutineRemarks = (from i in _context.TableA.Include(a => a.pm_routine_report_type)
from j in _context.TableB.Include(a => a.PM_Evt_Cat).Include(b => b.department).Include(c => c.employees).Include(d => d.provncs)
orderby i.seen_by_executive_on descending
orderby j.English_seen_by_executive_on descending
// Here i face the problem, i want to select i+j
select i+j).ToList();
At the end it allows me to only select either i or j, but i want to select both, how can i do that?
Try in this way
select new {I=i, J=j}).ToList();
I also agree with #GertArnold. it is most likely that your main query needs join, but it is hard to tell what you need to do without knowing your ERD
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.
I am learning about LINQ-to-SQL and everything was going well until something strange happened:
I tried to make an example of distinct, so, using the Northwind dabatase I wrote the following query:
var query =
from o in db.Orders
orderby o.CustomerID
select new
{
o.CustomerID
};
If I print the SQL generated by LINQ-to-SQL for the query stored in query it looks like this:
SELECT [t0].[CustomerID]
FROM [dbo].[Orders] AS [t0]
ORDER BY [t0].[CustomerID]
So, as usual, the query brings all the CustomerID for each Order in the Orders table ordered alphabetically.
But! If I use the Distinct() method like this:
var query = (
from o in db.Orders
orderby o.CustomerID
select new
{
o.CustomerID
}).Distinct();
The query brings the expected results of the Distinct clause, but the CustomerIDs are not ordered despite I wrote orderby o.CustomerID!
The SQL query for this second LINQ query is the following:
SELECT DISTINCT [t0].[CustomerID]
FROM [dbo].[Orders] AS [t0]
As we can see **the ORDER BY clause is missing. Why is that?
Why does the ORDER BY clause disappears when I use the Distinct() method?
From the Queryable.Distinct documentation;
The expected behavior is that it returns an unordered sequence of the unique items in source.
In other words, any order the existing IQueryable has is lost when you use Distinct() on it.
What you want is probably something more like this, an OrderBy() after the Distinct() is done;
var query = (from o in db.Orders
select new
{
o.CustomerID
}).Distinct().OrderBy(x => x.CustomerID);
Try rearranging the members to place the OrderBy after the Distinct. You'll have to revert to method chaining:
db.Orders.Select(o=>o.CustomerId).Distinct().OrderBy(id=>id);
This would be the more efficient way to set up the query in Enumerable Linq anyway, because the OrderBy would then operate only on the unique items and not on all of them. Also, according to MSDN, Enumerable.Distinct does not guarantee the return order of the elements anyway, so ordering before deduping is pointless.
Due to the use of distinct, the order of the returned list is not guaranteed. LinqToSql is smart enough to recognize this, therefor it ignores it.
If you place the order by AFTER your Distinct, everything will happen as you desire.
var query = (from o in db.Orders
select new
{
o.CustomerID
}).Distinct().OrderBy(o => o.CustomerID);
or
var query = db.Orders.Select(o => o.CustomerID).Distinct().OrderBy(o => o.CustomerID);
Please see this article for clarification:
http://programminglinq.com/blogs/marcorusso/archive/2008/07/20/use-of-distinct-and-orderby-in-linq.aspx
You can simulate ORDERBY and DISTINCT with this counstruction:
var distinctItems = employees.GroupBy(x => x.EmpID).OrderBy(x => x).Select(y => y.First());
I have a table, lets call it Record. Containing:
ID (int) | CustID (int) | Time (datetime) | Data (varchar)
I need the latest (most recent) record for each customer:
SQL
select * from record as i group by i.custid having max(id);
LINQ version 1
dgvLatestDistinctRec.DataSource = from g in ee.Records
group g by g.CustID into grp
select grp.LastOrDefault();
This throws an error:
System.NotSupportedException was unhandled by user code Message=LINQ
to Entities does not recognize the method 'Faizan_Kazi_Utils.Record
LastOrDefault[Record
](System.Collections.Generic.IEnumerable`1[Faizan_Kazi_Utils.Record
])' method, and this method cannot be translated into a store
expression. Source=System.Data.Entity
LINQ version 2
var list = (from g in ee.Records
group g by g.CustID into grp
select grp).ToList();
Record[] list2 = (from grp in list
select grp.LastOrDefault()).ToArray();
dgvLatestDistinctRec.DataSource = list2;
This works, but is inefficient because it loads ALL records from the database into memory and then extracts just the last (most recent member) of each group.
Is there any LINQ solution that approaches the efficiency and readability of the mentioned SQL solution?
Update:
var results = (from rec in Record group rec by rec.CustID into grp
select new
{
CustID = grp.Key,
ID = grp.OrderByDescending(r => r.ID).Select(x => x.ID).FirstOrDefault(),
Data = grp.OrderByDescending(r => r.ID).Select(x => x.Data).FirstOrDefault()
}
);
So I made a test table and wrote a Linq -> SQL Query that will do exactly what you need. Take a look at this and let me know what you think. Only thing to keep in mind if this query is scaled I believe it will run a query to the DB for each and every CustID record after the grouping in the select new. The only way to be sure would be to run SQL Tracer when you run the query for info on that go here .. http://www.foliotek.com/devblog/tuning-sql-server-for-programmers/
Original:
Could you do something like this? from g in ee.Records where g.CustID == (from x in ee.Records where (g.CustID == x.CustID) && (g.ID == x.Max(ID)).Select(r => r.CustID))
That's all pseudo code but hopefully you get the idea.
I'm probably too late to help with your problem, but I had a similar issue and was able to get the desired results with a query like this:
from g in ee.Records
group g by g.CustID into grp
from last in (from custRec in grp where custRec.Id == grp.Max(cr => cr.Id) select custRec)
select last
What if you replace LastOrDefault() with simple Last()?
(Yes, you will have to check your records table isn't empty)
Because I can't see a way how MySQL can return you "Default" group. This is not the thing that can be simply translated to SQL.
I think grp.LastOrDefault(), a C# function, is something that SQL doesn't know about. LINQ turns your query into an SQL query for your db server to understand. You might want to try and create an stored procedure instead, or another way to filter out what your looking for.
The reason your second query works is because the LINQ to SQL returns a list and then you do a LINQ query (to filter out what you need) on a C# list, which implements the IEnumerable/IQueryable interfaces and understands the grp.LastOrDefault().
I had another idea:
// Get a list of all the id's i need by:
// grouping by CustID, and then selecting Max ID from each group.
var distinctLatest = (from x in ee.Records
group x by x.CustID into grp
select grp.Max(g => g.id)).ToArray();
// List<Record> result = new List<Record>();
//now we can retrieve individual records using the ID's retrieved above
// foreach (int i in distinctLatest)
// {
// var res = from g in ee.Records where g.id == i select g;
// var arr = res.ToArray();
// result.Add(res.First());
// }
// alternate version of foreach
dgvLatestDistinctRec.DataSource = from g in ee.Records
join i in distinctLatest
on g.id equals i
select g;
I had tried to join two table conditionally but it is giving me syntax error. I tried to find solution in the net but i cannot find how to do conditional join with condition. The only other alternative is to get the value first from one table and make a query again.
I just want to confirm if there is any other way to do conditional join with linq.
Here is my code, I am trying to find all position that is equal or lower than me. Basically I want to get my peers and subordinates.
from e in entity.M_Employee
join p in entity.M_Position on e.PostionId >= p.PositionId
select p;
You can't do that with a LINQ joins - LINQ only supports equijoins. However, you can do this:
var query = from e in entity.M_Employee
from p in entity.M_Position
where e.PostionId >= p.PositionId
select p;
Or a slightly alternative but equivalent approach:
var query = entity.M_Employee
.SelectMany(e => entity.M_Position
.Where(p => e.PostionId >= p.PositionId));
Following:
from e in entity.M_Employee
from p in entity.M_Position.Where(p => e.PostionId >= p.PositionId)
select p;
will produce exactly the same SQL you are after (INNER JOIN Position P ON E..PostionId >= P.PositionId).
var currentDetails = from c in customers
group c by new { c.Name, c.Authed } into g
where g.Key.Authed == "True"
select g.OrderByDescending(t => t.EffectiveDate).First();
var currentAndUnauthorised = (from c in customers
join cd in currentDetails
on c.Name equals cd.Name
where c.EffectiveDate >= cd.EffectiveDate
select c).OrderBy(o => o.CoverId).ThenBy(o => o.EffectiveDate);
If you have a table of historic detail changes including authorisation status and effective date. The first query finds each customers current details and the second query adds all subsequent unauthorised detail changes in the table.
Hope this is helpful as it took me some time and help to get too.
I have an SQL Query as given below
SELECT ui.PageStyleCss
FROM UserImages ui
WHERE ui.UserImageId IN
( SELECT inv.UserImageId
FROM Invitation inv
JOIN InviteeEmails invEmails ON
inv.InviteID = invEmails.InviteID
WHERE invEmails.InviteGUID = #InviteGUID
)
How can I write this in LINQ?
Thanks
My wild guess is that you're using LINQ to SQL. It would be nice if you mentioned this, along with details of your model. Guessing at its structure...
var q = from ui in Context.UserImages
where ui.Invitations.Any(i => i.InviteeEmails.Any(e => e.InviteGuid = inviteGuid))
select ui.PageStyleCss;
from ui in db.UserImages
where (from inv in db.Invitations
join invEmails from InviteeEmails
on inv.InviteId equals invEmails.InviteId
where invEmails.InviteGUID == inviteGUID
select inv.UserImageId).Contains(ui.UserImageId)
select ui.PageStyleCss
(not sure if it compiles or not)
I have to assume there's a better way...this is pretty much a direct translation.