Question

here my problem, i’m using asp.net for my site, i get a list of books and show them, all is ok. now i try to sort this books by title, author or other attribute, i’m using this code:

public static IDataReader GetPageByCritere(
        int pageNumber,
        int pageSize,
        out int totalPages,
        string critere,
        string direction)
    {
        int pageLowerBound = (pageSize * pageNumber) - pageSize;
        totalPages = 1;
        int totalRows = GetCount();

        if (pageSize > 0) totalPages = totalRows / pageSize;

        if (totalRows <= pageSize)
        {
            totalPages = 1;
        }
        else
        {
            int remainder;
            Math.DivRem(totalRows, pageSize, out remainder);
            if (remainder > 0)
            {
                totalPages += 1;
            }
        }

        StringBuilder sqlCommand = new StringBuilder();
        sqlCommand.Append("SELECT * ");
        sqlCommand.Append("FROM a_book ");
        //sqlCommand.Append("WHERE  ");
        sqlCommand.Append("ORDER BY ?Critere ?direction ");
        sqlCommand.Append("LIMIT ?PageSize ");

        if (pageNumber > 1)
        {
            sqlCommand.Append("OFFSET ?OffsetRows ");
        }

        sqlCommand.Append(";");

        MySqlParameter[] arParams = new MySqlParameter[4];

        arParams[0] = new MySqlParameter("?PageSize", MySqlDbType.Int32);
        arParams[0].Direction = ParameterDirection.Input;
        arParams[0].Value = pageSize;

        arParams[1] = new MySqlParameter("?OffsetRows", MySqlDbType.Int32);
        arParams[1].Direction = ParameterDirection.Input;
        arParams[1].Value = pageLowerBound;

        arParams[2] = new MySqlParameter("?Critere", MySqlDbType.VarChar, 50);
        arParams[2].Direction = ParameterDirection.Input;
        arParams[2].Value = critere;

        arParams[3] = new MySqlParameter("?direction", MySqlDbType.VarChar, 50);
        arParams[3].Direction = ParameterDirection.Input;
        arParams[3].Value = direction;

        return MySqlHelper.ExecuteReader(
            GetReadConnectionString(),
            sqlCommand.ToString(),
            arParams);
    }
}

when i execute this code the books don’t get sorted, i get the first items list not sorted, here my sqlCommand and arParams for sorting with title for exemple :
sqlCommand

 {SELECT * FROM a_book ORDER BY ?Critere ?direction LIMIT ?PageSize ;}

arParams :

 {?PageSize} : 20 {?OffsetRows} : -20 {?Critere} title {?direction} DESC

please help, i didn’t find any solution.

Answer 1

Unfortunately, this used to work in MySql but then the behaviour changed – see http://bugs.mysql.com/bug.php?id=31474. It now behaves like Oracle and SQL Server in this respect.

Instead, you might want to consider building up your order by based on the criteria – hopefully, your critere string is not entered by a user or sourced from user input otherwise you’ll have to guard against SQL Injection.

If your criteria consists of a single column, then you can simply do something like:

sqlCommand.Append(String.Format("ORDER BY {0} {1}", Critere, direction))

If Critere is multi columned then you might want to consider passing your sorting criteria as a list of column-direction.

This is also covered here: Parameter in order by clause doesn't order -mysql, C#