LINQ gurus, I am looking for help to write a query...
I have a table with Person records, and it has a nullable ParentID column, so it is kind of self-referencing, where each record might have a Parent.
I am looking for unprocessed rows whose parent rows were processed.
This SQL works fine:
SELECT *
FROM Person
where IsProcessed = 0 and
ParentId in
(
select Id from Person
where IsProcessed = 1
)
I tried a number of LINQ queries, but they failed. Now, I'm trying:
var qParent =
from parent in db.Person
where
parent.IsProcessed == true
select parent.ID;
var qChildren = from child in db.Person
where
child.IsProcessed == false
&& child.ParentId.HasValue
select child.ParentId.Value;
var q2 = qChildren.Intersect(qParent);
This yields SQL with a DISTINCT clause, for some reason, and I am baffled why DISTINCT is generated.
My main question is how to write LINQ for the SQL statement above?
Thanks in advance.
Intersect is a set operation - it is meant to return a set of distinct elements from the intersection. It seems reasonable to me that it would use DISTINCT in the SQL. There could be multiple children with the same parent, for example - Intersect should only return that ID once.
Is there any reason you don't want to use a join here?
var query = from parent in db.Person
where parent.IsProcessed
join child in db.Person.Where(child => !child.IsProcessed)
on parent.ID equals child.ParentId.Value
select child;
The query can be translated literally into :
var parentIds = db.Person.Where(x => x.IsProcessed)
.Select(x => x.Id)
.ToList();
var result = db.Person.Where(x => !x.IsProcessed && parentIds.Contains(x => x.Id))
.ToList();
Related
I am trying to convert the following sql query to LINQ statement
SELECT t.*
FROM (
SELECT Unique_Id, MAX(Version) mversion
FROM test
GROUP BY Unique_Id
) m INNER JOIN
test t ON m.Unique_Id = t.Unique_Id AND m.mversion = t.Version
LINQ statement
var testalt = (from altt in CS.test
group altt by altt.Unique_Id into g
join bp in CS.alerts on g.FirstOrDefault().Unique_Id equals bp.Unique_Id
select new ABCBE
{
ABCName= bp.Name,
number = bp.Number,
Unique_Id = g.Key,
Version = g.Max(x=>x.Version)
});
I am getting an error of where clause. Please help
SQL FIDDLE
This is not an easy straight forward conversion but you can accomplish the same thing using linq method syntax. The first query is executed to an expression tree, then you are joining that expression tree from the grouping against CS.alerts. This combines the expression tree from CS.test query into the expression tree of CS.alerts to join the two expression trees.
The expression tree is evaluated to build the query and execute said query upon enumeration. Enumeration in this case is the ToList() call but anything that gets a result from the enumeration will execute the query.
var query1 = CS.test.GroupBy(x => x.Unique_Id);
var joinResult = CS.alerts.Join(query1,
alert => new { ID = alert.Unique_Id, Version = alert.Version },
test => new { ID = test.Key, Version = test.Max(y => y.Version },
(alert, test) => new ABCBE {
ABCName = alert.Name,
number = alert.Number,
Unique_Id = test.Key,
Version = test.Max(y => y.Version)
}).ToList();
Because query1 is still an IQueryable and you are using CS.alerts (which I'm guessing CS is your data context) it should join and build the query to execute upon the ToList() enumeration.
How can I write this SQL statement in LINQ ?
Select top 1 1 from MyTable
where some condition
or something like,
Select Top 1 1 from table1 inner join table2 on x=y where condition
First()/Take() returns first record. Please let me know if these functions solves the problem and how.
How about that:
var result = context.MyTable.Select(x => 1).FirstOrDefault();
with WHERE:
var result = context.MyTable.Where(x => true).Select(x => 1).FirstOrDefault();
Just add Select(x => 1).FirstOrDefault() at the end of the LINQ query.
I have a history table for Students in SQL Server 2008.
StudentHistoryId, StudentId, Grade, CreatedAt, ModifiedAt, ModifiedBy, Active
I am new to LINQ.
How do I write a LINQ query to get the latest modified row for all the active students and also the equivalent sql query for the same ?
Something like (Assuming LINQ-SQL):
using (YourDataContext db = new YourDataContext())
{
var data = from s in db.Students
select new
{
StudentId = s.StudentId,
LastHistory = s.Histories
.OrderByDescending(s => s.ModifiedAt)
.Where(s => s.Active)
.FirstOrDefault()
};
}
This is assuming that you want all students, regardless of whether they actually have any history. If don't want this, you can start with the History table and group by Student ID.
To view the SQL, you can hover the variable in debugging to see the SQL produced. I'm too lazy to convert the LINQ ;-)
var q =
from h in history
where h.Active
group h by new { h.StudentId, h.Grade } into g
select new
{
StudentId = g.Key.StudentId,
Grade = g.Key.Grade,
LatestModified = g.Max (x => x.ModifiedAt)
}
LINQ
var tempresult = (from student in Students
where Active == true).OrderByDesc(ModifiedAt)
List<Student> results = new List<Student>();
foreach(var result in tempResult)
{
if((results.Where(r => r.StudentId == result.StudentId).FirstOrDefault()) == null)
{
results.Add(result);
}
}
SQL
Select [Rows]
From Students S
Where S.Active = 1
And S.ModifiedAt = (Select Max(ModifiedAt)
From Student S1
Where S1.StudentId = S.StudentId)
The Linq is hacky (and I'm sure there's a better way, but I can't remember it) and I'm only sort-of confident about the SQL syntax (though that should point you in the right direction even if it's not exactly right), but either of those should get: The maximum ModifiedAt for every student that is currently active.
.FirstOrDefault() [LINQ] or Top 1 would only select the single row (only one student) with the most recent ModifiedAt.
I am using LINQ to SQL to retrieve data, using boolean conditions (BIT columns in SQL). My LINQ query looks something like this:
var query = from r in db.Requests
select r;
query = query.Where(r => r.Completed == someBooleanVal);
query = query.Where(r => r.Cancelled == someOtherBool);
return query.ToList();
The 'Where()' gets applied in a different method, that's why I'm putting it in separately.
When the boolean values are given as false, the generated SQL looks something like this:
SELECT [t0].[col1], [t0].[col2], [t0].[col3], [t0].[etc]
FROM [dbo].[Requests] AS [t0]
WHERE (NOT(([t0].[Cancelled]) = 1) AND (NOT(([t0].[Completed]) = 1)
in stead of what I would use:
WHERE [t0].[Cancelled] = 0 AND [t0].[Completed] = 0
This runs very, very slowly. I strongly suspect that it is because of the negative conditions on the boolean values it generated (all the selected columns are covered by an index, and the two columns in the where clause have a separate index on them).
Why is it generating negative conditions? How can I fix it?
var query =
from r in db.Requests.Where(r => r.Completed == someBooleanVal && r.Cancelled == someOtherBool)
select r;
return query.ToList();
Hope it can help you and have a nice day.
How do I do this
Select top 10 Foo from MyTable
in Linq to SQL?
Use the Take method:
var foo = (from t in MyTable
select t.Foo).Take(10);
In VB LINQ has a take expression:
Dim foo = From t in MyTable _
Take 10 _
Select t.Foo
From the documentation:
Take<TSource> enumerates source and yields elements until count elements have been yielded or source contains no more elements. If count exceeds the number of elements in source, all elements of source are returned.
In VB:
from m in MyTable
take 10
select m.Foo
This assumes that MyTable implements IQueryable. You may have to access that through a DataContext or some other provider.
It also assumes that Foo is a column in MyTable that gets mapped to a property name.
See http://blogs.msdn.com/vbteam/archive/2008/01/08/converting-sql-to-linq-part-7-union-top-subqueries-bill-horst.aspx for more detail.
Use the Take(int n) method:
var q = query.Take(10);
The OP actually mentioned offset as well, so for ex. if you'd like to get the items from 30 to 60, you would do:
var foo = (From t In MyTable
Select t.Foo).Skip(30).Take(30);
Use the "Skip" method for offset.
Use the "Take" method for limit.
#Janei: my first comment here is about your sample ;)
I think if you do like this, you want to take 4, then applying the sort on these 4.
var dados = from d in dc.tbl_News.Take(4)
orderby d.idNews descending
select new
{
d.idNews,
d.titleNews,
d.textNews,
d.dateNews,
d.imgNewsThumb
};
Different than sorting whole tbl_News by idNews descending and then taking 4
var dados = (from d in dc.tbl_News
orderby d.idNews descending
select new
{
d.idNews,
d.titleNews,
d.textNews,
d.dateNews,
d.imgNewsThumb
}).Take(4);
no ? results may be different.
This works well in C#
var q = from m in MyTable.Take(10)
select m.Foo
Whether the take happens on the client or in the db depends on where you apply the take operator. If you apply it before you enumerate the query (i.e. before you use it in a foreach or convert it to a collection) the take will result in the "top n" SQL operator being sent to the db. You can see this if you run SQL profiler. If you apply the take after enumerating the query it will happen on the client, as LINQ will have had to retrieve the data from the database for you to enumerate through it
I do like this:
var dados = from d in dc.tbl_News.Take(4)
orderby d.idNews descending
select new
{
d.idNews,
d.titleNews,
d.textNews,
d.dateNews,
d.imgNewsThumb
};
You would use the Take(N) method.
Taking data of DataBase without sorting is the same as random take
Array oList = ((from m in dc.Reviews
join n in dc.Users on m.authorID equals n.userID
orderby m.createdDate descending
where m.foodID == _id
select new
{
authorID = m.authorID,
createdDate = m.createdDate,
review = m.review1,
author = n.username,
profileImgUrl = n.profileImgUrl
}).Take(2)).ToArray();
I had to use Take(n) method, then transform to list, Worked like a charm:
var listTest = (from x in table1
join y in table2
on x.field1 equals y.field1
orderby x.id descending
select new tempList()
{
field1 = y.field1,
active = x.active
}).Take(10).ToList();
This way it worked for me:
var noticias = from n in db.Noticias.Take(6)
where n.Atv == 1
orderby n.DatHorLan descending
select n;