/** append a SQL string to the query field. Return the added SQL.
	 * As of version 2.1 this method assumes paramerized queries.
	* @param string columnName The name of the column. If the name needs to be prefixed, it must already be prefixed.
	* @param mixed $value 
	* @param mixed $placeholder to use (parameterized queries)
	*/
	function where_equals($columnName, $value, $placeholder='?') {
		$sql = " WHERE ($columnName ";
		$sql .= $value === null ? "IS " : "= ";
		$sql .= $this->param($value, $placeholder);  // $this->convertConditionArgumentToSql($value);
		$sql .= ')';
	
		$this->query .= $sql;
		
		return $sql;
	}