mysql complex joins within a table : I have a mysql session table with below entries
sessionid conversationid agentid Reason 
  s1          c1            a1    r1
  s2          c1            a1    r2
  s3          c2            a2    r1
  s4          c3            a3    r2

i want sesionid for the rows where agentid and conversationid's are same and first occurance of reason is r1. 

Expected output:
  sessionid conversationid agentid Reason 
  s1          c1            a1    r1
  s2          c1            a1    r2


Kinly help.