@Override
  public void meet(Count node) throws RuntimeException {
    builder.append("COUNT(");

    if (node.isDistinct()) {
      builder.append("DISTINCT ");
    }

    if (node.getArg() == null) {
      // this is a weird special case where we need to expand to all variables selected in the query
      // wrapped
      // by the group; we cannot simply use "*" because the concept of variables is a different one
      // in SQL,
      // so instead we construct an ARRAY of the bindings of all variables

      List<String> countVariables = new ArrayList<>();
      for (SQLVariable v : parent.getVariables().values()) {
        if (v.getProjectionType() == ValueType.NONE) {
          Preconditions.checkState(
              v.getExpressions().size() > 0, "no expressions available for variable");

          countVariables.add(v.getExpressions().get(0));
        }
      }
      builder.append("ARRAY[");
      Joiner.on(',').appendTo(builder, countVariables);
      builder.append("]");

    } else {
      optypes.push(ValueType.NODE);
      node.getArg().visit(this);
      optypes.pop();
    }
    builder.append(")");
  }
  @Override
  public void meet(Var node) throws RuntimeException {
    // distinguish between the case where the variable is plain and the variable is bound
    SQLVariable sv = parent.getVariables().get(node.getName());

    if (sv == null) {
      builder.append("NULL");
    } else if (sv.getBindings().size() > 0) {
      // in case the variable is actually an alias for an expression, we evaluate that expression
      // instead, effectively replacing the
      // variable occurrence with its value
      sv.getBindings().get(0).visit(this);
    } else {
      String var = sv.getAlias();

      if (sv.getProjectionType() != ValueType.NODE && sv.getProjectionType() != ValueType.NONE) {
        // in case the variable represents a constructed or bound value instead of a node, we need
        // to
        // use the SQL expression as value; SQL should take care of proper casting...
        // TODO: explicit casting needed?
        builder.append(sv.getExpressions().get(0));
      } else {
        // in case the variable represents an entry from the NODES table (i.e. has been bound to a
        // node
        // in the database, we take the NODES alias and resolve to the correct column according to
        // the
        // operator type
        switch (optypes.peek()) {
          case STRING:
            Preconditions.checkState(var != null, "no alias available for variable");
            builder.append(var).append(".svalue");
            break;
          case INT:
            Preconditions.checkState(var != null, "no alias available for variable");
            builder.append(var).append(".ivalue");
            break;
          case DECIMAL:
          case DOUBLE:
            Preconditions.checkState(var != null, "no alias available for variable");
            builder.append(var).append(".dvalue");
            break;
          case BOOL:
            Preconditions.checkState(var != null, "no alias available for variable");
            builder.append(var).append(".bvalue");
            break;
          case DATE:
            Preconditions.checkState(var != null, "no alias available for variable");
            builder.append(var).append(".tvalue");
            break;
          case TZDATE:
            Preconditions.checkState(var != null, "no alias available for variable");
            builder.append(parent.getDialect().getDateTimeTZ(var));
            break;
          case URI:
            Preconditions.checkState(var != null, "no alias available for variable");
            builder.append(var).append(".svalue");
            break;
          case TERM:
          case NODE:
            if (sv.getExpressions().size() > 0) {
              // this allows us to avoid joins with the nodes table for simple expressions that only
              // need the ID
              builder.append(sv.getExpressions().get(0));
            } else {
              Preconditions.checkState(var != null, "no alias available for variable");
              builder.append(var).append(".id");
            }
            break;
        }
      }
    }
  }