从Access数据库中删除不存在的记录

Question

I use the below code to DELETE a row in a table when a specific string(entity) is found. It works fine but it fails if the "entity" does not exist in the database.

using (OleDbConnection thisConnection = new OleDbConnection(connectionname))
{
    string deletequery = " DELETE FROM SFModelOutputVariables WHERE [Entity] = '" + ENTITY + "'";  
    OleDbCommand myAccessCommandDelete = new OleDbCommand(deletequery, thisConnection);
    try
    {
        thisConnection.Open();
    }
    catch(Exception ex)
    {
        MessageBox.Show(ex.Message.ToString() + "\n" + "-Error found while " + "connecting to Access Database");
        return;
    }

    myAccessCommandDelete.ExecuteNonQuery();
    thisConnection.Close();
}

I get an error for the line myAccessCommandDelete.ExecuteNonQuery(); when data for the selected entity does not exist in the table.

Answer 1

You need to check if the entity exsists. Run the delete if the entity exsist but if it doesnt exsist possibly return a 404 to the user or display an appropriate message.

    using (OleDbConnection thisConnection = new OleDbConnection(connectionname))
    {

 string cmdStr = "Select count(*) from SFModelOutputVariables WHERE [Entity] = '" + ENTITY + "'"; 

 OleDbCommand cmd = new      OleDbCommand(cmdStr, thisConnection);

   int count = (int)cmd.ExecuteScalar();

   if(count == 0)
   {
         MessageBox.Show("Sorry no entity was found :-(");
            return;
   }

   // write your code for removing things here....
 }

Answer 2

You can use executescalar method, its giving row count of table. if its > 0 you can perform delete operations.

MSDN ExecuteScalar method sample