/// <summary>
/// 分组指定字段名并返回当前语句
/// </summary>
/// <typeparam name="T">实体类类型</typeparam>
/// <param name="cmd">查询语句</param>
/// <param name="expr">实体类属性</param>
/// <exception cref="ExpressionInvalidException">表达式不正确</exception>
/// <exception cref="NullAttributeException">没有设置特性</exception>
/// <returns>当前语句</returns>
/// <example>
/// <code lang="C#">
/// <![CDATA[
/// public class UserRepository : DatabaseTable<User>
/// {
/// //other necessary code
///
/// public DataTable GetAllEntities()
/// {
/// return this.Select()
/// .Querys<User>(c => new { c.UserType })
/// .GroupBy<User>(c => c.UserType)
/// .ToDataTable(this);
///
/// //SELECT UserType From tbl_Users GROUP BY UserType
/// }
/// }
/// ]]>
/// </code>
/// </example>
public static SelectCommand GroupBy <T>(this SelectCommand cmd, Expression <Func <T, Object> > expr)
{
DatabaseColumnAttribute attr = SelectCommandExtension.GetColumnAttribute(cmd, expr.Body);
return(cmd.GroupBy(attr.ColumnName));
}
