Seleccione la primera fila de cada grupo en sql

Puedes usar el ROW_NUMBER() funcionar así:

SELECT *
  FROM(SELECT workflowid, salesRepId, quantityAssigned,
              quantityLeft, month, year
              , ROW_NUMBER()
                OVER (PARTITION BY salesRepId
                          ORDER BY workflowid) AS rownumber
         FROM sm_salesRepWorkflow)
 WHERE rownumber = 1;

Demostración de violín