Distinct over one column with value comparing on another column ICriteria NHibernate - nhibernate

Question

I have table and object called Person. I have problem to create a distinct (over column "lastname") criteria. I want to get only the oldest Person with distinct lastnames.
For example i have (properties: firstname, lastname, age):
John Smith, 52
Jessica Smith, 45
Ann Pit, 21
Brad Pit, 30
Can anybody help me to create criteria which result i get Person object with John Smith and Brad Pit?

Answer 1

Probably the best approach here is to use EXISTS to filter the result set, first a SQL example to get the logic correct:
DECLARE #Person TABLE (
Id INT,
Firstname VARCHAR(20),
Lastname VARCHAR(20),
Age INT
)
INSERT INTO #Person VALUES (1, 'Brad', 'Pitt', 42)
INSERT INTO #Person VALUES (2, 'Angelina', 'Pitt', 45)
INSERT INTO #Person VALUES (3, 'John', 'Smith', 50)
INSERT INTO #Person VALUES (4, 'Jane', 'Smith', 55)
SELECT P.* FROM #Person P
WHERE EXISTS(
SELECT SUB.LastName, MAX(SUB.Age) as Age FROM #Person SUB
GROUP BY SUB.LastName
HAVING SUB.LastName = P.LastName AND MAX(SUB.Age) = P.Age)
This yields the following results which is as expected:
Id Firstname Lastname Age
-------------------------------
2 Angelina Pitt 45
4 Jane Smith 55
Now to convert to nHibernate, this effectively builds the same query as above:
var subQuery = DetachedCriteria.For<Person>("SUB")
.SetProjection(Projections.ProjectionList()
.Add(Projections.GroupProperty("LastName"), "Lastname")
.Add(Projections.Max("Age"), "Age"))
.Add(Restrictions.EqProperty(Projections.Max("Age"), "P.Age")
.Add(Restrictions.EqProperty("LastName", "P.LastName"));
return session.CreateCriteria<Person>("P")
.Add(Subqueries.Exists(subQuery))
.List<Person>();