SQL classification
1. select all sessions
2. for each session S
   I. prepare an accumulator ACC for clusters
   II. select the clusters of viewed documents for this session
   III. for each cluster C accumulate the cluster count ( ACC[C]++ )
   IV. find the maximum in the ACC. That is the cluster that the session was classified to

+-------+-----------+--------------------+
| ID    | sessionID | classified_cluster |
+-------+-----------+--------------------+

+-------+-----------+------------+
| ID    | sessionID | documentID |
+-------+-----------+------------+

+-------+-------+
| ID    | label |
+-------+-------+

+-------+-----------+------------+
| ID    | clusterID | documentID |
+-------+-----------+------------+

+-----------+-----------------------+
| sessionID | classifiedIntoCluster |
+-----------+-----------------------+

SELECT SD.session_id, CD.cluster_id
FROM cluster_document AS CD
INNER JOIN session_document AS SD
ON CD.document_id = SD.document_id
WHERE session_id IN (SELECT session_id FROM session)

private void ClassifyUsers() {
        int nClusters = Database.SelectClusterCount(); //get number of clusters
        DataSet sessions = Database.SelectSessions(); //get all sessions
        foreach (DataRow session in sessions.Tables[0].Rows) { //foreach session
           int[] acc = new int[nClusters]; //prepare an accumulator for each known cluster
           string s_id = session["session_id"].ToString();
           DataSet sessionClusters = Database.SelectSessionClusters(s_id); //get clusters for this session

           foreach (DataRow cluster in sessionClusters.Tables[0].Rows) { //for each cluster
               int c = Convert.ToInt32(cluster["cluster_id"].ToString()) - 1;
               acc[c]++; //accumulate the cluster count
           }

           //find the maximum in the accumulator -> that is the most relevant cluster
           int max = 0;
           for (int j = 0; j < acc.Length; j++) {
               if (acc[j] >= acc[max]) max = j;
           }
           max++;
           Database.UpdateSessionCluster(s_id, max); //update the session with its new assigned cluster
       }
    }

session id    |  cluster
abc                 0
def                 0
ghi                 0
jkl                 0
mno                 0

cluster_id  | label
1               A
2               B
3               C
4               D
5               E

id      | session_id    |   document_id
1           abc             1
2           def             5
3           jkl             3
4           ghi             4
5           mno             2
6           def             2
7           abc             5
8           ghi             3

id      | cluster_id    |   document_id
1           1                  2
2           1                  3
3           2                  5
4           3                  5
5           3                  1
6           4                  3
7           5                  2
8           5                  4

session_id  |  cluster_id   | label     | document_id
abc             3               C           1
abc             2               B           5
abc             3               C           5
-----
def             2               B           5
def             3               C           5
def             1               A           2
def             5               E           2
----
ghi             5               E           4
ghi             1               A           3
ghi             4               D           3
----
jkl             1               A           3
jkl             4               D           3
----
mno             1               A           2
mno             5               E           2

session_id |    cluster_id  | label |   occurrence
abc             3               C           2   <--- MAX
abc             2               B           1
----
def             2               B           1
def             3               C           1
def             1               A           1
def             5               E           1   <--- MAX
----
ghi             5               E           1
ghi             1               A           1
ghi             4               D           1   <--- MAX
----
jkl             1               A           1
jkl             4               D           1   <--- MAX
----
mno             1               A           1
mno             5               E           1   <--- MAX

session_id |    cluster_id
abc                 3
def                 5
ghi                 4
jkl                 4
mno                 5

+------------+------------+-------+------------+
| SESSION_ID | CLUSTER_ID | LABEL | OCCURRENCE |
+------------+------------+-------+------------+
| abc        |          3 | C     |          2 |
| def        |          1 | A     |          1 |
| def        |          2 | B     |          1 |
| def        |          3 | C     |          1 |
| def        |          5 | E     |          1 |
| ghi        |          1 | A     |          1 |
| ghi        |          4 | D     |          1 |
| ghi        |          5 | E     |          1 |
| jkl        |          1 | A     |          1 |
| jkl        |          4 | D     |          1 |
| mno        |          1 | A     |          1 |
| mno        |          5 | E     |          1 |
+------------+------------+-------+------------+

+------------+------------+
| SESSION_ID | CLUSTER_ID |
+------------+------------+
| abc        |          3 |
| def        |          5 |
| ghi        |          5 |
| jkl        |          4 |
| mno        |          5 |
+------------+------------+

select s1.session_id, max(s1.cluster_id) as cluster_id from (
  select sd.session_id, cd.cluster_id, count(*) as Occurrence
  from session_document sd
  join cluster_document cd
  on sd.document_id = cd.document_id
  join cluster c
  on c.cluster_id = cd.cluster_id
  group by sd.session_id, cd.cluster_id, c.label
) as s1
left join (
  select sd.session_id, count(*) as Occurrence
  from session_document sd
  join cluster_document cd
  on sd.document_id = cd.document_id
  join cluster c
  on c.cluster_id = cd.cluster_id
  group by sd.session_id, cd.cluster_id, c.label
) as s2
on s1.session_id = s2.session_id and s1.occurrence < s2.occurrence
where s2.occurrence is null
group by s1.session_id

create view MaxOccurrences as (
  select sd.session_id, cd.cluster_id, count(*) as Occurrence
  from session_document sd
  join cluster_document cd
  on sd.document_id = cd.document_id
  join cluster c
  on c.cluster_id = cd.cluster_id
  group by sd.session_id, cd.cluster_id, c.label
);

select s1.session_id, max(s1.cluster_id) as cluster_id
from MaxOccurrences as s1
left join MaxOccurrences as s2
on s1.session_id = s2.session_id and s1.occurrence < s2.occurrence
where s2.occurrence is null
group by s1.session_id

SELECT SESSION_ID,MAX(CNT) MAX_CNT
FROM (SELECT SD.SESSION_ID, CD.CLUSTER_ID,COUNT(*) AS CNT
FROM CLUSTER_DOCUMENT AS CD
INNER JOIN SESSION_DOCUMENT AS SD
ON CD.DOCUMENT_ID = SD.DOCUMENT_ID
GROUP BY SD.SESSION_ID,CD.CLUSTER_ID) CNT1
GROUP BY SESSION_ID

SELECT B.SESSION_ID, MAX(CNT2.CLUSTER_ID) FROM
(SELECT SESSION_ID,MAX(CNT) MAX_CNT
FROM (SELECT SD.SESSION_ID, CD.CLUSTER_ID,COUNT(*) AS CNT
FROM CLUSTER_DOCUMENT AS CD
INNER JOIN SESSION_DOCUMENT AS SD
ON CD.DOCUMENT_ID = SD.DOCUMENT_ID
GROUP BY SD.SESSION_ID,CD.CLUSTER_ID) CNT1
GROUP BY SESSION_ID) B
JOIN (SELECT SD.SESSION_ID, CD.CLUSTER_ID,COUNT(*) AS CNT
FROM CLUSTER_DOCUMENT AS CD
INNER JOIN SESSION_DOCUMENT AS SD
ON CD.DOCUMENT_ID = SD.DOCUMENT_ID
GROUP BY SD.SESSION_ID,CD.CLUSTER_ID) CNT2
ON B.SESSION_ID = CNT2.SESSION_ID
AND B.MAX_CNT = CNT2.CNT
GROUP BY B.SESSION_ID