docs: public void checkValidCheckConstraint() {
  
          HsqlArrayList set = new HsqlArrayList();
  
          Expression.collectAllExpressions(set, this, subqueryExpressionSet,
                                           emptyExpressionSet);
  
          if (!set.isEmpty()) {
              throw Error.error(ErrorCode.X_0A000,
                                "subquery in check constraint");
          }
      }