private JoinRelType convertJoinType(JoinType joinType) {
            switch (joinType) {
                case INNER:
                    return JoinRelType.INNER;
                case LEFT_OUTER:
                    return JoinRelType.LEFT;
                case RIGHT_OUTER:
                    return JoinRelType.RIGHT;
                case FULL_OUTER:
                    return JoinRelType.FULL;
                default:
                    throw new TableException("Unknown join type: " + joinType);
            }
        }
Get the {@link AggCall} correspond to the aggregate or table aggregate expression.
core/src/main/java/org/apache/flink/table/planner/plan/QueryOperationConverter.java