title: Mysql self join for unique combinations
body: <p>I have a table session as below:</p>
  
  <pre><code>sessionid   conversationid  agentid
  
  SES153130074572578571   001a60f3-d95c-4434-a8c0-fabb7d815277_OD112807172056802000   igs.399760
  SES15313020119321989    001a60f3-d95c-4434-a8c0-fabb7d815277_OD112807172056802000   hgs.20073
  SES153130276854998136   001a60f3-d95c-4434-a8c0-fabb7d815277_OD112807172056802000   igs.100000000308235
  SES153128080803552749   00803e49-0325-4bef-b133-8c5e97f2fccc_OD112706138643350000   hgs.20031
  SES153128342589414965   00803e49-0325-4bef-b133-8c5e97f2fccc_OD112706138643350000   hgs.20031
  SES153129466185930775   008bba3a-98fa-414a-8f99-96956513bc66_OD112757403158502000   igs.401836
  SES153129612178429544   008bba3a-98fa-414a-8f99-96956513bc66_OD112757403158502000   hgs.20076
  SES153129179161527928   00976601-aac7-4ec7-9e03-61cd59875650_Payment Related Queries    hgs.20071
  SES153129238786010778   00976601-aac7-4ec7-9e03-61cd59875650_Payment Related Queries    igs.100000000307102
  </code></pre>
  
  <p>i want the rows where a conversationid is assigned to only one agent id (cases where one conversationid is assigned to more than one agentid would be omitted)
      i.e. my output should be as below:</p>
  
  <pre><code>sessionid   conversationid  agentid
  SES153128080803552749   00803e49-0325-4bef-b133-8c5e97f2fccc_OD112706138643350000   hgs.20031
  SES153128342589414965   00803e49-0325-4bef-b133-8c5e97f2fccc_OD112706138643350000   hgs.20031
  </code></pre>
  
  <p>Kindly help with the query</p>