Select row with max value in multiple where condition - sql-server

Question

I edited the following query based on this page:
Selecting a Record With MAX Value
Select query :
select
Users.Id, Users.[Name], Users.Family, Users.BirthDate,
Users.Mobile, Users.[Description], Users.Email,
Users.UserName, Users.fatherName,
Users.archiveNumber, Users.[Address], Users.IsMarried,
Users.Mazhab,
Cities.CityName, Religions.PersianName, Users.Date_insert,
Users.ImageName,
MaghtaeTahsilis.[Name] as MaghtaeTahsilisName,
FieldStudies.[Name] as FieldStudiesName,
Eductionals.Institute, Eductionals.Moaddal,
Eductionals.FromYear, Eductionals.ToYear
from
Users
left outer join
Eductionals on Users.id = Eductionals.UserID
left outer join
MaghtaeTahsilis on Eductionals.MaghtaeID = MaghtaeTahsilis.ID
left outer join
Cities on Users.City_Id = Cities.Id
left outer join
Religions on Users.Relegion_ID = Religions.ID
left outer join
FieldStudies on Eductionals.FieldStudy_ID = FieldStudies.ID
where
Users.UserName = #code_melli
and Eductionals.MaghtaeID = (select MAX(MaghtaeID) from Eductionals
where Eductionals.UserID = Users.Id)
This command works correctly in choosing MAX value, But if the following statement has a NULL value, no row are returned. I want to show NULL value if it is NULL.

Answer 1

Your left outer joins are being turned into inner joins by the where conditions. Your query should look like:
select u.Id, u.[Name], u.Family, u.BirthDate, u.Mobile, u.[Description], u.Email, u.UserName, u.fatherName,
u.archiveNumber, u.[Address], u.IsMarried, u.Mazhab, c.CityName, r.PersianName, u.Date_insert, u.ImageName,
mt.[Name] As MaghtaeTahsilisName, fs.[Name] As FieldStudiesName, e.Institute, e.Moaddal, e.FromYear, e.ToYear
from Users u left outer join
Eductionals e
on u.id = e.UserID and
e.MaghtaeID = (select MAX(e2.MaghtaeID)
from Eductionals e2
where e2.UserID = u.Id
) left outer join
MaghtaeTahsilis mt
on e.MaghtaeID = mt.ID left outer join
Cities c
on u.City_Id = c.Id left outer join
Religions r
on u.Relegion_ID = r.ID left outer join
FieldStudies fs
on e.FieldStudy_ID = fs.ID
where u.UserName = #code_melli ;
Conditions on the first table -- in a chain of left joins should be in the where clause. On subsequent tables in the on clauses.
You'll notice that I also added table aliases so the query is easier to write and to read.
You can also use window functions:
from Users u left outer join
(select e2.*,
row_number() over (partition by e2.userId order by e2.MaghtaeID desc) as seqnum
from Eductionals e2
) e
on u.id = e.UserID and
e.seqnum = 1 left outer join
. . .

Answer 2

Reason for returning zero records when second query returns NULL is, when second query returns NULL, your SQL syntax become like this
And Eductionals.MaghtaeID=NULL
And probably Dbtable Educationals holds NULL values for field MaghtaeID.
So SQL fails above syntax and thus returns zero records.
Correct syntax for checking NULL values would be
And Eductionals.MaghtaeID is NULL
So please modify where condition in your query as follows which will return desired result.
where Users.UserName = #code_melli AND isnull(Eductionals.MaghtaeID,0) = isnull((select MAX(MaghtaeID) from Eductionals where Eductionals.UserID = Users.Id),0)