to get unique records from table after join

Question

I hav tables event , session , sessionspeaker , speaker and Employee event table has EventID as PK , session table can have multiple Eventid and has sessionid as pk , speaker table has speakerid as pk and has empID column, employee table has empID as PK . sessionspeaker table has (sessionid, speakerid) as pk. Now I wud like to hav all the eventid with Topmost emplID such that the event id is unique

SELECT     emp.EmployeeID, e.EventID
FROM    Event AS e INNER JOIN Session ON e.EventID = Session.EventID left outer JOIN
SessionSpeaker ON Session.SessionID = SessionSpeaker.SessionID INNER JOIN
Speaker ON SessionSpeaker.EmployeeID = Speaker.EmployeeID INNER JOIN
Employee AS emp ON Speaker.EmployeeID = emp.EmployeeID 

this query returns

EmployeeID   EventID
5351         310
5351         310
5352         299
5352         300
5352         301
5352         302
5352             303
5352         314
5352         315

this is what I don't want, I want unique event ID and topmost employeeID Kindly, let me know an easy way to solve this. will groupby, having or count help in this matter

Answer 1

I want unique event ID and topmost employeeID

Just GROUP BY e.EventID and select MAX(EmployeeId) :

SELECT 
  e.EventID, 
  MAX(emp.EmployeeID) AS TopMostEmpId, 
FROM    Event AS e 
INNER JOIN Session         ON e.EventID                 = Session.EventID 
INNER JOIN SessionSpeaker  ON Session.SessionID         = SessionSpeaker.SessionID 
INNER JOIN Speaker         ON SessionSpeaker.EmployeeID = Speaker.EmployeeID 
INNER JOIN Employee AS emp ON Speaker.EmployeeID        = emp.EmployeeID
GROUP BY e.EventID

If you want to select more columns than eventId , employeeId you can use a ranking function something like this:

WITH CTE AS
(
    SELECT 
      e.EventID, 
      emp.EmployeeID,
      e.EventName,
      emp.EmployeeName,
      ...
      ROW_NUMBER() OVER(PARTITION BY e.EventID 
                        ORDER BY emp.EmployeeId DESC) AS RN
    FROM    Event AS e 
    INNER JOIN Session         ON e.EventID                 = Session.EventID 
    INNER JOIN SessionSpeaker  ON Session.SessionID         = SessionSpeaker.SessionID 
    INNER JOIN Speaker         ON SessionSpeaker.EmployeeID = Speaker.EmployeeID 
    INNER JOIN Employee AS emp ON Speaker.EmployeeID        = emp.EmployeeID
) 
SELECT *
FROM CTE
WHERE RN = 1;

Answer 2

SELECT     emp.EmployeeID, distinct e.EventID
FROM    Event AS e INNER JOIN Session ON e.EventID = Session.EventID left outer JOIN
SessionSpeaker ON Session.SessionID = SessionSpeaker.SessionID INNER JOIN
Speaker ON SessionSpeaker.EmployeeID = Speaker.EmployeeID INNER JOIN
Employee AS emp ON Speaker.EmployeeID = emp.EmployeeID 

group by e.EventID