code: public static JoinRelType toJoinRelType(FlinkJoinType joinType) {
  		switch (joinType) {
  			case INNER:
  				return JoinRelType.INNER;
  			case LEFT:
  				return JoinRelType.LEFT;
  			case RIGHT:
  				return JoinRelType.RIGHT;
  			case FULL:
  				return JoinRelType.FULL;
  			default:
  				throw new IllegalArgumentException("invalid: " + joinType);
  		}
  	}
docs: Converts {@link FlinkJoinType} to {@link JoinRelType}.