第二个查询不运行

Question

I am validating existance of a record before it inserts new record in. However the code does not jump into insert block. it jumps to finally block after reader.close

i suspect i must be doing something wrong as there is no exceptions and the recordsaffected =-1 every time

{
    // check if data exists before inserting
    command.Connection = connection;
    command.CommandType = CommandType.Text;
    command.CommandText = " SELECT id from  [dbo].[trade_events]  where  instrument =@instrument and datetime>@datetime and eventname=@eventname";
    command.Parameters.AddWithValue("@datetime", dt);
    command.Parameters.AddWithValue("@instrument", instrument);
    command.Parameters.AddWithValue("@eventname", eventname);
    connection.Open();
    SqlDataReader reader = command.ExecuteReader();
    while (reader.Read()) ;
    reader.Close();
    if (reader.RecordsAffected <= 0)
    {
        // <== insert data now

        command.CommandText = " INSERT INTO [dbo].[trade_events]  ([datetime] ,[instrument],[source],[eventname],[eventdetails],[state],[account],[connection]) " +
                              "values(@datetime, @instrument,@source, @eventname,@eventdetails,@state,@account,@connection)";

        //  command.CommandText = "UPDATE [dbo].[Alerts] SET [datetimealertrecieved] = @timerecieved where  instrument=@instrument and alertType =@alerttype and  datetimealertrecieved is null" ;

        command.Parameters.AddWithValue("@datetime", dt);
        command.Parameters.AddWithValue("@instrument", instrument);
        command.Parameters.AddWithValue("@source", source);
        command.Parameters.AddWithValue("@eventname", eventname);
        command.Parameters.AddWithValue("@eventdetails", eventdetails);
        command.Parameters.AddWithValue("@state", state);
        command.Parameters.AddWithValue("@account", account);
        command.Parameters.AddWithValue("@connection", dataconnection);

        connection.Open();
        int recordsAffected = command.ExecuteNonQuery();
    }

}
catch (SqlException ex)
{
    // error here
    var notice = airbrake.BuildNotice(ex);
    var response = airbrake.NotifyAsync(notice).Result;
    notice = airbrake.BuildNotice(command.CommandText);
    response = airbrake.NotifyAsync(notice).Result;
}
finally
{
    connection.Close();
}

Answer 1

If you look at the documentation of RecordsAffected it's clear why you get -1:

The number of rows changed, inserted, or deleted; 0 if no rows were affected or the statement failed; and -1 for SELECT statements .

You can use the HasRows property:

using(SqlDataReader reader = command.ExecuteReader())
{
    if (reader.HasRows)
    {
        // insert
    }
}

Answer 2

You have to clear previous parameters before adding the new ones:

var cont=false;
  while (reader.Read())

{
....your code
 cont=true ;
}
reader.Close();

if (cont)
{

command.Parameters.Clear();

 command.Parameters.AddWithValue("@datetime", dt);
        command.Parameters.AddWithValue("@instrument", instrument);
        command.Parameters.AddWithValue("@source", source);
        command.Parameters.AddWithValue("@eventname", eventname);
.....
}

Answer 3

reader.RecordsAffected isn't reliable, and isn't intended for this, ultimately; if you're trying to test for existence, well... the "least changes to the code" way would be to increment a counter to count the rows :

int records = 0;
while (reader.Read()) records++;
if (records == 0) {...}

or perhaps the more lazy:

bool any = reader.Read();
if (any) {...}

but: that's unnecessarily noisy; you could select count(1) from... , but even that is an extra round trip. An OK-ish idea might be the single SQL operation (that does the existence and insert in the same batch):

if not exists (
    select id from  [dbo].[trade_events]
    where  instrument =@instrument and datetime>@datetime and eventname=@eventname
)
begin
    insert into ...
end

but even this is still a race condition, unless you have suitable transactional isolation.