从SQL表获取数据到空变量

Question

I'm trying to search in table(called Accounts) for Username and in the line of this user in coulmn 4 that called PhotoId there is string that i need to get and add it to empty string value that i made before. thats what i tryied i want to do it by Query like my code

string ecid = e.CallbackQuery.Message.Chat.Id.ToString();
using (SqlCommand sqlCommand = new SqlCommand("SELECT * from Accounts where Username like @username", con))
{
    con.Open();
    sqlCommand.Parameters.AddWithValue("@username", ecid);
    string variable;
    string userfound = (string)sqlCommand.ExecuteScalar();
    con.Close();
    if (userfound == ecid)
    {
        using (SqlCommand GotPhoto = new SqlCommand("SELECT PhotoId from Accounts where Username like @user", con))
        {
            con.Open();
            GotPhoto.Parameters.AddWithValue("@user", ecid);

            DataPid = GotPhoto.ExecuteScalarAsync().ToString();
            con.Close();

            //  await bot.SendPhotoAsync("xxx", DataPid, capo, parseMode: ParseMode.Markdown, replyMarkup: zz);

            await bot.SendTextMessageAsync(cid, "its been Poster", parseMode: ParseMode.Html);
        }
    }
    else
    {
        MessageBox.Show("Not Working");
    }
}

Answer 1

You are combining a SELECT * ... with ExecuteScalar() , which "returns the value of the first column in the first row of the resultset. Additional rows and columns are ignored."( https://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlcommand.executescalar(v=vs.110).aspx ). Probably username is not the first column in the accounts table, so the content of another column is retured and your comparison fails.

Use SELECT username from Accounts where Username like @username instead.

EDIT

Depending on your data-model you can also combine the two queries into one as you are querying the same row twice.

con.Open();
using (var cmd = new SqlCommand("SELECT PhotoId from Accounts where Username like @user", con)) {
    cmd.Parameters.AddWithValue("@user", ecid);
    using (var reader = await cmd.ExecuteReaderAsync()) {
        if (!reader.HasRows || ! reader.Read()) {
            MessageBox.Show("User not found");
        } else {
            DataPid = cmd.GetYOURDATATYPE(0).ToString();
            //....
        }

    }
}
con.Close();

Answer 2

According to the documentation of ExecuteScalar =>

Executes the query, and returns the first column of the first row in the result set returned by the query. Additional columns or rows are ignored.

When you perform string userfound = (string)sqlCommand.ExecuteScalar();

You are comparing the username to a value ( but wich one ? the value of the first column in the create table instruction)

I Suggest you change the first query to be sure to select username

PS:

• To be consistant, you can also rename both of your query parameter @username instead of @user and @username because it refers exactly to the same thing.
• You can perform only one query instead of two. ( select photoid ... where username = .. , and just check if the result contains one row )