I have struggling to run linq left on multiple tables.
tableA
Select all the (courseID, code, title) from courseInstances table
tableB
select (studyLevel_ID) from Courses table where courseID from tableA = CourseID from tableB. tableB has courseID
tableC
Select (StudyLevelDescription) from StudyLevel table where studyLevelID from tableB = studyLevel from tableC.
I believe I need left join on table A as I need all the records
I have done separate linq which are working fine but struggling to bring combine result
CourseInstances results
var processedCourseInstance =
(from _courseInstances in _uof.CourseInstances_Repository.GetAll()
join _courses in _uof.Courses_Repository.GetAll() on _courseInstances.CourseID equals _courses.CourseID
into a from b in a.DefaultIfEmpty()
orderby _courseInstances.CourseCode
select _courseInstances).ToList();
StudyLevel results for each course
var _CoursesStudyLevel_Lookup =
(from _course in _uof.Courses_Repository.GetAll()
join _studyLevel in _uof.StudyLevel_Repository.GetAll() on _course.StudyLevelId equals _studyLevel.StudyLevelID
select new {_course.CourseID, _course.StudyLevelId, _studyLevel.LevelDescription }).ToList();
I have managed to combine two results but NOT with LEFT join on CourseInstance table. This time I used LINQPad
from _courseInstances in CourseInstances
join _courses in Courses on _courseInstances.CourseID equals _courses.CourseID
join _studylevel in StudyLevels on _courses.StudyLevelId equals _studylevel.StudyLevelID
orderby _courseInstances.CourseCode
select new {_courseInstances.CourseID, _courseInstances.CourseCode, _courseInstances.CourseTitle, _courseInstances.UCASCode, _courses.StudyLevelId, _studylevel.LevelDescription, _studylevel.SLevelType }
for above SQL version as following;
SELECT [t0].[CourseID], [t0].[CourseCode], [t0].[CourseTitle],
[t0].[UCASCode], [t1].[StudyLevelId], [t2].[LevelDescription], [t2].[SLevelType]
FROM [CourseInstances] AS [t0]
INNER JOIN [Courses] AS [t1] ON [t0].[CourseID] = ([t1].[CourseID])
INNER JOIN [StudyLevel] AS [t2] ON [t1].[StudyLevelId] = ([t2].[StudyLevelID])
ORDER BY [t0].[CourseCode]
If i understand correctly, you want something like this?
from _courseInstances in CourseInstances
join c in Courses on _courseInstances.CourseID equals c.CourseID into courses
from _courses in courses.DefaultIfEmpty()
join sl in StudyLevels on _courses.StudyLevelId equals sl.StudyLevelID into studyLevels
from _studylevel in studyLevels.DefaultIfEmtpy()
orderby _courseInstances.CourseCode
select new {
_courseInstances.CourseID,
_courseInstances.CourseCode,
_courseInstances.CourseTitle,
_courseInstances.UCASCode,
_courses.StudyLevelId,
_studylevel.LevelDescription,
_studylevel.SLevelType
}
}
You can create a LINQ left join query with the into keyword and .DefaultIfEmpt().
Related
So I have a SQL query that I would like to convert to LINQ.
Here is said query:
SELECT *
FROM DatabaseA.SchemaA.TableA ta
LEFT OUTER JOIN DatabaseA.SchemaA.TableB tb
ON tb.ShipId = ta.ShipId
INNER JOIN DatabaseA.SchemaA.TableC tc
ON tc.PostageId= tb.PostageId
WHERE tc.PostageCode = 'Package'
AND ta.MailId = 'Specification'
The problem I am struggling with is I cannot seem to figure out how to do a left join in LINQ before an inner join, since doing a left join in LINQ is not as clear to me at least.
I have found numerous examples of a LINQ inner join and then a left join, but not left join and then inner join.
If it helps, here is the LINQ query I have been playing around with:
var query = from m in tableA
join s in tableB on m.ShipId equals s.ShipId into queryDetails
from qd in queryDetails.DefaultIfEmpty()
join p in tableC on qd.PostageId equals p.PostageId
where m.MailId == "Specification" && p.PostageCode == "Package"
select m.MailId;
I have tried this a few different ways but I keep getting an "Object reference not set to an instance of an object" error on qd.PostageId.
LINQ is very new to me and I love learning it, so any help on this would be much appreciated. Thanks!
From my SQL conversion recipe:
JOIN conditions that aren't all equality tests with AND must be handled using where clauses outside the join, or with cross product (from ... from ...) and then where
JOIN conditions that are multiple ANDed equality tests between the two tables should be translated into anonymous objects
LEFT JOIN is simulated by using into joinvariable and doing another from from the joinvariable followed by .DefaultIfEmpty().
The order of JOIN clauses doesn't change how you translate them:
var ans = from ta in TableA
join tb in TableB on ta.ShipId equals tb.ShipId into tbj
from tb in tbj.DefaultIfEmpty()
join tc in TableC on tb.PostageId equals tc.PostageId
where tc.PostageCode == "Package" && ta.MailId == "Specification"
select new { ta, tb, tc };
However, because the LEFT JOIN is executed before the INNER JOIN and then the NULL PostageIds in TableB for unmatched rows will never match any row in TableC, it becomes equivalent to an INNER JOIN as well, which translates as:
var ans2 = from ta in tableA
join tb in tableB on ta.ShipId equals tb.ShipId
join tc in tableC on tb.PostageId equals tc.PostageId
where tc.PostageCode == "Package" && ta.MailId == "Specification"
select new { ta, tb, tc };
Use:
var query = from m in tableA
join s in tableB on m.ShipId equals s.ShipId
join p in tableC on s.PostageId equals p.PostageId
where m.MailId == "Specification" && p.PostageCode == "Package"
select m.MailId;
Your query uses a LEFT OUTER JOIN but it doesn't need it.
It will, in practice, function as an INNER JOIN due to your tc.PostageCode = 'Package' clause. If you compare to a column value in a table in a WHERE clause (and there are no OR clauses and you aren't comparing to NULL) then effectively all joins to get to that table will be treated as INNER).
That clause will never be true if TableB is null (which is why you use LEFT OUTER JOIN vs INNER JOIN) - so you should just use an INNER JOIN to make the problem simpler.
I've got the following LINQ Query
from s in db.tblSave
join a in db.tblAssessment on s.AssessmentID equals a.Id
join staff in db.tblStaff on s.StaffID equals staff.Id
join student in db.tblStudent on s.StudentID equals student.Id
join signed in db.tblSaveSigned on s.Id equals signed.SaveID
select new
{
SaveID = s.Id,
StaffName = staff.StaffName,
AssessmentName = a.AssessmentName,
StudentName= student.StudentName,
CreatedDate = s.CreatedDate,
SignedDate = signed.SignedDate}
tblSaveSigned may not have a record against tblSave which is excluding some tblSave records. How do I include all tblSave records in my results? It's an outer join in SQL but not sure how to do it in LINQ.
Thanks in advance.
Is there an SQL to LINQ tool?
Below is the correct LINQ Query for put FULL OUTER JOIN on tblSave for tblSaveSigned. Thanks for NetMage for pointing me in the right direction.
from s in db.tblSave
join a in db.tblAssessment on s.AssessmentID equals a.Id
join staff in db.tblStaff on s.StaffID equals staff.Id
join student in db.tblStudent on s.StudentID equals student.Id
join signed in db.tblSaveSigned on s.Id equals signed.SaveID into signedj
from signed in signedj.DefaultIfEmpty()
select new
{
SaveID = s.Id,
StaffName = staff.StaffName,
AssessmentName = a.AssessmentName,
StudentName= student.StudentName,
CreatedDate = s.CreatedDate,
SignedDate = signed.SignedDate}
Let's say I have this query in SQL:
SELECT
....
FROM
TableA a
JOIN TableB b on a.Id = b.Id
I've got that working just fine as a linq query:
var results = (from a in db.TableA
join b in db.TableB on a.Id equals b.Id
select new MyObject {...}).ToList();
But now I want to add a left join to the mix as well. In SQL:
SELECT
....
FROM
TableA a
JOIN TableB b on a.Id = b.Id
LEFT JOIN TableC c on a.Id2 = c.Id2
I'm not sure how to handle this in the linq query.
SQL's LEFT JOIN concept is a pretty clunky abstraction when you think about what you're really trying to do. You really want, for every TableA entry, to have a list of TableC items associated with it, right?
LINQ uses a much simpler (IMO) way to represent this abstraction, because it has the concept of nested, hierarchical objects. The provider will generate a LEFT JOIN, but it'll also build the result into an object structure that's easy to work with.
var results = (from a in db.TableA
join b in db.TableB on a.Id equals b.Id
select new MyObject {
...
TableCEntries =
from c in db.TableC
where c.Id == a.Id
select c // or select specific values from c
}).ToList();
If you really want your results flattened out the way they would be with a left outer join, you could do this:
var results = (from a in db.TableA
join b in db.TableB on a.Id equals b.Id
from c in db.TableC
where c.Id == a.Id
select new MyObject {
...
}).ToList();
... but you wouldn't get any entries for TableA where there are no entries present for TableC. For that behavior you'd have to do ugly stuff with group by/DefaultIfEmpty as shown in Ivan Stoev's link.
I have three tables below:
table patient
{ NID *pk
Name
Family
}
table disease
{
ICD_code *pk
Title
}
table_patient_disease *-*
{
ID
Fk_ICDcode
FK_Patient
}
it is a n-m relationship between patient and disease.
i want to write a query to select patients and their diseases
it needs to join in linq
the Sql pure query is:
SELECT
dbo.Table_Disease.*,
dbo.Table_PatDis.*,
dbo.Table_Patient.*
FROM
dbo.Table_Disease
INNER JOIN dbo.Table_PatDis ON dbo.Table_Disease.ICD_code = dbo.Table_PatDis.FK_Disease
INNER JOIN dbo.Table_Patient ON dbo.Table_PatDis.FK_PAtient = dbo.Table_Patient.NID
what is the same Linq statement ?
SELECT
dbo.Table_Disease.*,
dbo.Table_PatDis.*,
dbo.Table_Patient.*
FROM
dbo.Table_Disease
INNER JOIN dbo.Table_PatDis ON dbo.Table_Disease.ICD_code = dbo.Table_PatDis.FK_Disease
INNER JOIN dbo.Table_Patient ON dbo.Table_PatDis.FK_PAtient = dbo.Table_Patient.NID
Would become:
var results = (from d in DbContext.Table_Disease
join pd in DbContext.Table_PatDis on d.ICD_Code equals pd.FK_Disease
join p in DbContext.Table_Patient on pd.FK_PAtient equals p.NID
select new {d, pd, p});
How to use leftouter join in entity framework query
I have two table one is item and other is stock available I want to get all items and also get its quantity from stock available table which depend upon the particular department
For example
LINQ Query
var query = (from p in dc.GetTable<Person>()
join pa in dc.GetTable<PersonAddress>() on p.Id equals pa.PersonId into tempAddresses
from addresses in tempAddresses.DefaultIfEmpty()
select new { p.FirstName, p.LastName, addresses.State });
SQL Translation
SELECT [t0].[FirstName], [t0].[LastName], [t1].[State] AS [State]
FROM [dbo].[Person] AS [t0]
LEFT OUTER JOIN [dbo].[PersonAddress] AS [t1] ON [t0].[Id] = [t1].[PersonID]