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