i just learning LINQ. so first of all i need to be familiar with join with linq. i search google for left outer and right outer join with linq and i got answer like
left outer join
var LeftJoin = from emp in ListOfEmployees
join dept in ListOfDepartment
on emp.DeptID equals dept.ID into JoinedEmpDept
from dept in JoinedEmpDept.DefaultIfEmpty()
select new
{
EmployeeName = emp.Name,
DepartmentName = dept != null ? dept.Name : null
};
right outer join
var RightJoin = from dept in ListOfDepartment
join employee in ListOfEmployees
on dept.ID equals employee.DeptID into joinDeptEmp
from employee in joinDeptEmp.DefaultIfEmpty()
select new
{
EmployeeName = employee != null ? employee.Name : null,
DepartmentName = dept.Name
};
from then code i just could not understand how it is left outer join because no left outer key word is use here. so please tell me how to understand that the join is left outer join or right outer.
when i will use linq then how like operator can be use. 'a%' or '%a' or '%a%'. i saw there is contain method which is bit different.
please discuss the two issue. thanks
The "join ... in ... on ... into" piece of LINQ query syntax, is translated into a GroupJoin().
GroupJoin() method, for each key in the outer list (or table), returns a list of elements in the inner list (or table) having the same key, or an empty list if such key doesn't exist.
Hence, the left outer join code of your question is clearer:
If JoinedEmpDept (i.e. the list of elements having the same key of the current examined outer list entry) is empty, dept is set to null (thanks to DefaultIfEmpty() method).
Translation in pseudo code:
for each employee in ListOfEmployees
get the list of dept having ID equal to empl.DeptID
and set them into JoinedEmpDept
then for each dept in JoinedEmpDept
(if empty iterates over a single null dept)
returns an new element containing:
employee.Name and dept.Name (or null if dept is null)
The right outer join instead, is basically a left outer join with outer and inner lists exchanged.
About the "like" question, you should use string.Contains("a") for '%a%', string.StartsWith("a") for 'a%', string.EndsWith("a") for '%a'
Example:
var query = from el in listOfStrings
where el.StartsWith("AB")
select el;
EDIT:
About the IN() operator question...
well, you can use Contains() also for that, or Any():
var inGroup = new []{ "Foo", "Bar" };
var query1 = from el in listOfStrings
where inGroup.Contains(el)
select el;
// or equally
var query2 = from el in listOfStrings
where inGroup.Any(x => el.Equals(x))
select el;
The left outer join is so because of this line:
from dept in JoinedEmpDept.DefaultIfEmpty()
which will get all of the employees, even if they are not in a department. The DefaultIfEmpty turns the join into an left outer join, when the SQL is generated.
See this blog post for more details: C#: Left outer joins with LINQ
Left join Tip,
Instead of:
from user in tblUsers
join compTmp1 in tblCompanies
on user.fkCompanyID equals compTmp1.pkCompanyID into compTmp2
from comp in compTmp2.DefaultIfEmpty()
You can write:
from user in tblUsers
from comp in tblCompanies.Where(c => c.pkCompanyID == user.fkCompanyID).DefaultIfEmpty()
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.
This is my Query in SQL :
Select distinct * from tr.Table1
Left Outer join tr.Table2 on tr.Table1.ID = tr.Table2.ID
Left Outer join tr.Table3 on tr.Table2.AId= tr.Table3.ID
where tr.Table1.Deleted =1 and tr.Table1.Ready=1 and tr.Table1.Show=0
The query is working in SQL and gives the expected results.The thing here is that I want the equivalent of this using LINQ. I have tried some variations in LINQ queries such as :
var query = from p in _ctx.Table1
join s in _ctx.Table2 on p.Id equals s.Id into bag1
from to in bag1.DefaultIfEmpty()
join tx in _ctx.Table3 on to.AId equals tx.Id into bag2
from ts in bag2.DefaultIfEmpty()
select new
{
ContactNo = to.Table1.ContactNo
};
But it always doesn't return all the field values. Some are returned as NULL. Also tried referring to some other link as well but they all focus on joining with the parent table whereas I have to join one of the joined tables with the other one. So here I am, struggling with this.
This is the output that I'm getting as of now. Some values are null. The field has values but due to some joining issue, they are returned as NULL.
Guidance here is appreciated. Thank you.
Your query looks fine to me, the reason why you must be getting the Nulls is because when we use DefaultIfEmpty, it returns null for non-matching rows, thus you need to handle that while fetching the actual results. Try doing something like this:-
var query = from p in _ctx.Table1
join s in _ctx.Table2 on p.Id equals s.Id into bag1
from to in bag1.DefaultIfEmpty()
join tx in _ctx.Table3 on to.AId equals tx.Id into bag2
from ts in bag2.DefaultIfEmpty()
select new
{
ContactNo = to == null ? String.Empty : to.Table1.ContactNo
};
Assuming, ContactNo to be of type String, I have used String.Empty you can use any default value.
var query = from section in load_sections.Sections
join course in load_sections.Courses
on section.Course_Id equals course.Course_Id
join faculty in load_sections.Faculties
on section.Faculty_Id equals faculty.Faculty_Id
select section;
I have some null values in my section.Faculty_Id which will not be equal to any row in faculty.Faculty_Id and it is just returning the records where section.Faculty_Id is not null...If section.Faculty_Id is not null, then it must return the other remaining fields of Table Courses
If you can't drop the join on faculty for whatever reason, you'll have to construct an outer join:
var query = from section in load_sections.Sections
join course in load_sections.Courses
on section.Course_Id equals course.Course_Id
join faculty in load_sections.Faculties
on section.Faculty_Id equals faculty.Faculty_Id into faculties
from f in faculties.DefaultIfEmpty()
select section;
This executes a GroupJoin with Faculties. The effect of the subsequent from f in faculties is that the grouping is flattened again by a SelectMany. .DefaultIfEmpty() creates the outer join.
T-SQL:
declare #postlocations table (locationid int)
insert into #postlocations
select locationid
from dbo.PostLocations
where PostId = 162172
select t.*
from dbo.Themes t
inner join dbo.ThemeLocations tl on t.ThemeId = tl.ThemeId
inner join #postlocations pl on tl.LocationId = pl.locationid
LINQ-Entities i have so far:
var postLocations = e.SomePost.Locations; // pre-fetched, e.g materialized ICollection<Post>
var themes = (from t in db.Themes
join q in postLocations on t.Locations.Select(l => l.LocationId) equals q.LocationId
select t).ToList();
But the compiler is complaining on the join keyword about not being able to infer the type arguments.
Any ideas?
I don't think you can join a SQL table with an in-memory list of objects, even if those objects are originally from the database.
Convert the in-memory list of objects to a list of id's (integer), and use that in the join or in a Contains/sub-select. EF can translate the list of id's to parameters when generating the SQL.
The problem with your join is that you're implying a collection of LocationId (t.Locations.Select(l => l.LocationId) can equal a single LocationId. You're trying to join a Theme which has a collection of Locations onto a single Location.
You should be able to fix this by using Contains
var themes = (from t in db.Themes
join q in postLocations
on t.Locations.Select(l => l.LocationId).Contains(q.LocationId)
select t).ToList();
or if EF complains about passing a postLocations as a parameter, you can try
// I'd materialize this but you may not have to
var postLocationIds = postLocations.Select(p => p.LocationId).ToList();
var themes = db.Themes.Where(t => t.Locations.Any(l =>
postLocationIds.Contains(l.LocationId))).ToList();
Edit
how about this
///your sql query
select t.* from dbo.Themes t
inner join dbo.ThemeLocations tl on t.ThemeId = tl.ThemeId
inner join #postlocations pl on tl.LocationId = pl.locationid
//linq query for that
from t in teams
join from tl in teamlocation on t.themid = tl.ThemeID
join from pl in postlocation on tl.temeid = pl.temeid
select t;
Org
Not sure but you can try out by using let keyword
var themes = (from t in db.Themes
let location = t.Locations
join q in postLocations on location.LocationId equals q.LocationId
select t).ToList();
Id like to perform an outer join with the second join statement in this query, I keep getting weird errors! (it must be the 3rd RedBull)
var Objeto = from t in Table1.All()
join su in table2.All() on t.Id equals su.Id
join tab2 in Table1.All() on t.PId equals tab2.Id //<-I want it here
select new
{
t.Field1,
SN = su.Field123,
PTN = tab2.FieldABC
};
Any help would be appreciated.
[Edit] - I neglected to say that I'm using SubSonic 3.0, the bug seems to be with SubSonic.....
Performing an outer join requires two steps:
Convert the join into a group join with into
Use DefaultIfEmpty() on the group to generate the null value you expect if the joined result set is empty.
You will also need to add a null check to your select.
var Objeto = from t in Table1.All()
join su in table2.All() on t.Id equals su.Id
join tab2 in Table1.All() on t.PId equals tab2.Id into gj
from j in gj.DefaultIfEmpty()
select new
{
t.Field1,
SN = su.Field123,
PTN = (j == null ? null : j.FieldABC)
};