Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/ResultSetColumnHandle.java
* and they are:
	 * <ul>
	 * <li>COLUMN_DATA_TYPE_ANY
	 * <li>COLUMN_DATA_TYPE_INTEGER
	 * <li>COLUMN_DATA_TYPE_STRING
	 * <li>COLUMN_DATA_TYPE_DATETIME
	 * <li>COLUMN_DATA_TYPE_DECIMAL
	 * <li>COLUMN_DATA_TYPE_FLOAT
	 * <li>COLUMN_DATA_TYPE_STRUCTURE
	 * <li>COLUMN_DATA_TYPE_TABLE
	 * </ul>
	 * 
	 * @param dataType
	 *            the data type to set
	 * @throws SemanticException
	 *             if the dataType is not in the choice list.
	 */
	public void setDataType( String dataType ) throws SemanticException
	{
		setProperty( ResultSetColumn.DATA_TYPE_MEMBER, dataType );
	}
	/**
	 * Returns the position that this column is in the result set.
	 * 
	 * @return the position that this column is in the result set.
	 */
	public Integer getPosition( )
	{
		return (Integer) getProperty( ResultSetColumn.POSITION_MEMBER );
	}