MySql Fatal error when using auto serial number

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C#

MySQL

Hello people.

My first foray into MySQL after Ms Sql.

In C# and MySQL, I get results when I execute this method so no problem.

public DataTable SupplierPeople(String pSupplierId)
{
    String cmd = "Select perName As `Name`, speDesignation As `Designation`, speId As `SysId` From main.SupplierPeople Inner Join main.People On spePerId = perId Where speSupId = " + pSupplierId + ";";
    DataTable dat = new DataTable();
    dl = new DataLayer(cmd);
    if (dl.ConnectToServer())
    {
        dat = dl.ExecuteFillDataTable();
        dl.CloseConnection();
    }
    return dat;
}

But I need the serial number there so I put in somethings before the "Select" and it gives me an error "Fatal error encountered during command execution."

public DataTable SupplierPeople(String pSupplierId)
     {
         String cmd = "set @rn=0;Select @rn:=@rn+1 As `Sno`, perName As `Name`, speDesignation As `Designation`, speId As `SysId` From main.SupplierPeople Inner Join main.people On spePerId = perId Where speSupId = " + pSupplierId + ";";
         DataTable dat = new DataTable();
         dl = new DataLayer(cmd);
         if (dl.ConnectToServer())
         {
             dat = dl.ExecuteFillDataTable();
             dl.CloseConnection();
         }
         return dat;
     }

I tried running the cmd (capturing it at runtime) in Workbench and it works fine but not in C# program. Fyi, it's a Windows application.

Please help. Thank you.