code: private ICriteria addSimpleExpression(Restrictor criterion) {
  		SimpleRestrictor e = (SimpleRestrictor) criterion;
  		String restriction = e.getRestriction();
  		if (this.hasAlias()) {
  			queryString.append(this.alias).append(".");
  		}
  		queryString.append(e.getProperty());
  		queryString.append(restriction);
  		if (restriction.equals(restriction_isNull) || restriction.equals(restriction_isNotNull)) {
  			return this;
  		}
  		
  		if (this.namedParam) {
  			queryString.append(VALUE_NAMED_SIGN).append(e.getProperty());
  			namedValues.put(e.getProperty().trim(), e.getValue());
  		} else {
  			queryString.append(VALUE_PLACEHOLDER);
  			values.add(e.getValue());
  		}
  		return this;
  	}
docs: /*
  添加简单运算表达式