Linq - unable to join table due to null value - linq

Question

I am having problem with joining of multiple tables.
PatientID can be null. Whenever patientID is null, it is unable to join and the result will not be added to the list.
Any suggestion on how i can do to join them ?
List<ViewActivityLogViewModel> q = new List<ViewActivityLogViewModel>();
q = (from l in logs
join lc in logCategories on l.logCategoryID equals lc.logCategoryID
join p in patients on l.patientID equals p.patientID
join u in users on l.userIDInit equals u.userID
join ut in usertypes on u.userTypeID equals ut.userTypeID
join u2 in users on l.userIDApproved equals u2.userID
join ut2 in usertypes on u2.userTypeID equals ut2.userTypeID
select new ViewActivityLogViewModel
{
logID = l.logID,
logCategoryName =lc.logCategoryName,
firstName = p.firstName,
userIDInitfirstName = u.firstName,
userIDInituserTypeName = ut.userTypeName,
userIDApprovedfirstName = u2.firstName,
userIDApproveduserTypeName = ut2.userTypeName
}
).OrderBy(x=>x.createDateTime).ToList();
This is the answer i need. I written it in SQL.
SELECT l.logID, lc.logCategoryName,p.firstName, p.lastName, u.firstName, ut.userTypeName, u2.firstName, ut2.userTypeName
FROM log AS l
LEFT JOIN patient AS p ON l.patientID = p.patientID
JOIN logCategory AS lc ON l.logCategoryID = lc.logCategoryID
JOIN user AS u ON l.userIDInit = u.userID
JOIN userType AS ut ON u.userTypeID = ut.userTypeID
JOIN user AS u2 ON l.userIDApproved = u2.userID
JOIN userType AS ut2 ON u2.userTypeID = ut2.userTypeID

Answer 1

You can use GroupJoin to join the patients table and get the data. According to MSDN, GroupJoin in Lambda supports the LEFT JOIN in SQL.
GroupJoin preserves the order of the elements of outer, and for each
element of outer, the order of the matching elements from inner.
GroupJoin has no direct equivalent in
traditional relational database terms. However, this method does
implement a superset of inner joins and left outer joins. Both of
these operations can be written in terms of a grouped join.
So, your query can be written as,
var results =
logs.Join(logCategories, l => l.logCategoryID, lc => lc.logCategoryID, (l, lc) => new { l, lc })
.Join(users, llc => new { userId = llc.l.userIDInit }, u1 => new { userId = u1.userID }, (llc, u1) => new { llc.l, llc.lc, u1 })
.Join(userTypes, llcu1 => llcu1.u1.userTypeID, ut1 => ut1.userTypeID, (llcu1, ut1) => new { llcu1.l, llcu1.lc, llcu1.u1, ut1 })
.Join(users, llcu2 => new { approvedId = llcu2.l.userIDApproved }, u2 => new { approvedId = u2.userID }, (llcu2, u2) => new { llcu2.l, llcu2.lc, llcu2.u1, llcu2.ut1, u2 })
.Join(userTypes, llcu3 => llcu3.u2.userTypeID, ut2 => ut2.userTypeID, (llcu3, ut2) => new { llcu3.l, llcu3.lc, llcu3.u1, llcu3.u2, llcu3.ut1, ut2 })
.GroupJoin(patients, llcu4 => llcu4.l.patientID, p => p.patientID, (llcu4, p) => new { llcu4.l, llcu4.lc, llcu4.u1, llcu4.u2, llcu4.ut1, llcu4.ut2, p })
.SelectMany(x => x.p.DefaultIfEmpty(), (prj, y) =>
{
return new ViewActivityLogViewModel
{
logID = prj.l.logID,
logCategoryName = prj.lc.logCategoryName,
firstName = y != null ? y.firstName : null,
userIDInitfirstName = prj.u1.firstName,
userIDInituserTypeName = prj.ut1.userTypeName,
userIDApprovedfirstName = prj.u2.firstName,
userIDApproveduserTypeName = prj.ut2.userTypeName
};
}).ToList();
C# Fiddle with sample data.

Answer 2

I am not sure why did u
replicated the joins which could be achieved in two joins only
JOIN user AS u ON l.userIDInit = u.userID
JOIN userType AS ut ON u.userTypeID = ut.userTypeID
JOIN user AS u2 ON l.userIDApproved = u2.userID
JOIN userType AS ut2 ON u2.userTypeID = ut2.userTypeID
as
JOIN user AS u ON l.userIDInit = u.userID and l.userIDApproved = u.userID
JOIN userType AS ut ON u.userTypeID = ut.userTypeID
The below query will give u ur desired result
q = (from l in log
join lc in logCategory
on l.logCategoryID equals lc.logCategoryID
join u in user
on new {a1=l.userIDInit,a2=l.userIDApproved} equals
new {a1=u.userID, a2=u.userID}
join ut in userType
on u.userTypeID equals ut.userTypeID
join p in patient
on l.patientID equals p.patientID
into p11
from p1 in p11.DefaultIfEmpty()
select new {l,lc,u,ut,p1})
.Select(x=>new ViewActivityLogViewModel
{
logID = x.l.logID,
logCategoryName =x.lc.logCategoryName,
firstName = x.p1?.firstName,
userIDInitfirstName = x.u.firstName,
userIDInituserTypeName = x.ut.userTypeName,
userIDApprovedfirstName = x.u.firstName,
userIDApproveduserTypeName = x.ut.userTypeName
})
.OrderBy(x=>x.createDateTime).ToList();

Answer 3

You didn't say which PatientId can be null, so to be on the safe side I assume both can be null.
q = (from l in logs
join lc in logCategories.Where(category => category.PatientId != null)
on l.logCategoryID equals lc.logCategoryID
join p in patients where(patient => patien.PatiendId != null)
on l.patientID equals p.patientID
join ...