public JoinType JoinType {get; set;}

public async Task JoiningFileGeodatabaseFeatureClassToOracleQueryLayer()
{
  await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
  {
    using (Geodatabase geodatabase = new Geodatabase(new FileGeodatabaseConnectionPath(new Uri("C:\\Data\\LocalGovernment.gdb"))))
    using (Database database = new Database(new DatabaseConnectionProperties(EnterpriseDatabaseType.Oracle)
      {
        AuthenticationMode = AuthenticationMode.DBMS,
        Instance = "instance",
        User = "user",
        Password = "password",
        Database = "database"
      }))

    using (FeatureClass leftFeatureClass = geodatabase.OpenDataset<FeatureClass>("Hospital"))
    using (Table rightTable = database.OpenTable(database.GetQueryDescription("FacilitySite")))
    {
      Field originPrimaryKey = leftFeatureClass.GetDefinition().GetFields().FirstOrDefault(field => field.Name.Equals("facilityId"));
      Field destinationForeignKey = rightTable.GetDefinition().GetFields().FirstOrDefault(field => field.Name.Equals("hospitalID"));

      VirtualRelationshipClassDescription description = new VirtualRelationshipClassDescription(
        originPrimaryKey, destinationForeignKey, RelationshipCardinality.OneToOne);

      using (RelationshipClass relationshipClass = leftFeatureClass.RelateTo(rightTable, description))
      {
        JoinDescription joinDescription = new JoinDescription(relationshipClass)
        {
          JoinDirection = JoinDirection.Forward,
          JoinType = JoinType.LeftOuterJoin
        };

        Join join = new Join(joinDescription);

        using (Table joinedTable = join.GetJoinedTable())
        {
          // Perform operation on joined table.
        }
      }
    }
  });
}

public async Task JoinTablesFromDifferentGeodatabases()
{
  await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
  {
    using (Geodatabase sourceGeodatabase = new Geodatabase(new FileGeodatabaseConnectionPath(new Uri("Path \\ to \\Geodatabase \\ one"))))
    using (Geodatabase destinationGeodatabase = new Geodatabase(new FileGeodatabaseConnectionPath(new Uri("Path \\ to \\Geodatabase \\ two"))))
    using (Table sourceTable = sourceGeodatabase.OpenDataset<Table>("State"))
    using (Table destinationTable = destinationGeodatabase.OpenDataset<Table>("Cities"))
    {
      Field primaryKeyField = sourceTable.GetDefinition().GetFields().FirstOrDefault(field => field.Name.Equals("State.State_Abbreviation"));
      Field foreignKeyField = destinationTable.GetDefinition().GetFields().FirstOrDefault(field => field.Name.Equals("Cities.State"));

      VirtualRelationshipClassDescription virtualRelationshipClassDescription = new VirtualRelationshipClassDescription(primaryKeyField, foreignKeyField, RelationshipCardinality.OneToMany);

      using (RelationshipClass relationshipClass = sourceTable.RelateTo(destinationTable, virtualRelationshipClassDescription))
      {
        JoinDescription joinDescription = new JoinDescription(relationshipClass)
        {
          JoinDirection = JoinDirection.Forward,
          JoinType = JoinType.InnerJoin,
          TargetFields = sourceTable.GetDefinition().GetFields()
        };

        using (Join join = new Join(joinDescription))
        {
          Table joinedTable = join.GetJoinedTable();

          //Process the joined table. For example ..
          using (RowCursor cursor = joinedTable.Search())
          {
            while (cursor.MoveNext())
            {
              using (Row row = cursor.Current)
              {
                // Use Row
              }
            }
          }
        }
      }
    }
  });
}