presto-parser/src/main/java/io/prestosql/sql/parser/AstBuilder.java/AstBuilder/visitJoinRelation
class AstBuilder:    @Override
    public Node visitJoinRelation(SqlBaseParser.JoinRelationContext context)
    {
        Relation left = (Relation) visit(context.left);
        Relation right;

        if (context.CROSS() != null) {
            right = (Relation) visit(context.right);
            return new Join(getLocation(context), Join.Type.CROSS, left, right, Optional.empty());
        }

        JoinCriteria criteria;
        if (context.NATURAL() != null) {
            right = (Relation) visit(context.right);
            criteria = new NaturalJoin();
        }
        else {
            right = (Relation) visit(context.rightRelation);
            if (context.joinCriteria().ON() != null) {
                criteria = new JoinOn((Expression) visit(context.joinCriteria().booleanExpression()));
            }
            else if (context.joinCriteria().USING() != null) {
                criteria = new JoinUsing(visit(context.joinCriteria().identifier(), Identifier.class));
            }
            else {
                throw new IllegalArgumentException("Unsupported join criteria");
            }
        }

        Join.Type joinType;
        if (context.joinType().LEFT() != null) {
            joinType = Join.Type.LEFT;
        }
        else if (context.joinType().RIGHT() != null) {
            joinType = Join.Type.RIGHT;
        }
        else if (context.joinType().FULL() != null) {
            joinType = Join.Type.FULL;
        }
        else {
            joinType = Join.Type.INNER;
        }

        return new Join(getLocation(context), joinType, left, right, Optional.of(criteria));
    }