How to insert primary key from one table which created by auto increment, into a second table as forign key?

Question

This is part of my code, which I insert data into two tables. Tables primary key is defined as auto increment:: song table PK is song_id. file table PK is File_id. I want to insert into file table the song_id as FK. How should I do it? Thanks

try
{
    string MyConnection1 = "datasource=localhost;port=3306;username=root;password=123";
    string Query1 = "insert into myproject.song " +
                        "(song_name, house_number, song_text) " +
                      "values " +
                        "('" + line1 + "', '" + paragraphs.Length + 
                          "', '" + filetext + "');" + //Insert song name, song huose count and full song text
                    "insert into myproject.file " +
                        "(File_Location, Words_number, Lines_number, File_name, song_id) " +
                      "values " +
                        "('" + strfilename + "','" + words.Length + "','" + 
                          totallineCnt + "', '" + fileNameOnly + 
                          "', ?????????????;";  //Insert table file details                          
    MySqlConnection myConn1 = new MySqlConnection(MyConnection1);
    MySqlCommand MyCommand1 = new MySqlCommand(Query1, myConn1);
    MySqlDataReader MyReader1;
    myConn1.Open();
    MyReader1 = MyCommand1.ExecuteReader();
    while (MyReader1.Read())
    {
    }
    myConn1.Close();
}
catch (Exception ex)
{
    MessageBox.Show(ex.Message);
}

Answer 1

MySQL provides the LAST_INSERT_ID() function which returns the last value of an autoincrement column. To use it you would perform one INSERT , SELECT the function's value, and INSERT the returned value into the second table.

You should also be using prepared statements instead of dynamic SQL for a plethora of reasons which I won't go into here.

I'm not terribly comfortable with C#, but I'll give it a shot:

string MyConnection1 = "datasource=localhost;port=3306;username=root;password=123";
string Query1 = "insert into myproject.song " +
                    "(song_name, house_number, song_text) " +
                  "values " +
                    "(@SongName, @HouseNum, @SongText)"; //Insert song name, song huose count and full song full text

string Query2 = "SELECT LAST_INSERT_ID()";

string Query3 = "insert into myproject.file " +
                    "(File_Location, Words_number, Lines_number, File_name, song_id) " +
                  "values " +
                    "(@FileLoc, @WordCount, @LineCount, @FileName, @SongId";  //Insert the table file details                          

try
{
    MySqlConnection myConn1 = new MySqlConnection(MyConnection1);
    myConn1.Open();

    MySqlCommand Cmd1 = new MySqlCommand(Query1, myConn1);
    Cmd1.Parameters.AddWithValue("@SongName", line1);
    Cmd1.Parameters.AddWithValue("@HouseNum", paragraphs.Length);
    Cmd1.Parameters.AddWithValue("@SongText", filetext);
    Cmd1.ExecuteNonQuery();

    MySqlCommand Cmd2 = new MySqlCommand(Query2, myConn1);
    object result = Cmd2.ExecuteScalar();
    int songId = Convert.ToInt32(result);

    MySqlCommand Cmd3 = new MySqlCommand(Query3, myConn1);
    Cmd3.Parameters.AddWithValue("@FileLoc", strfilename);
    Cmd3.Parameters.AddWithValue("@WordCount", words.Length);
    Cmd3.Parameters.AddWithValue("@LineCount", totallineCnt);
    Cmd3.Parameters.AddWithValue("@FileName", fileNameOnly);
    Cmd3.Parameters.AddWithValue("@SongId", songId);
    Cmd3.ExecuteNonQuery();

    myConn1.Close();
}
catch (Exception ex)
{
    MessageBox.Show(ex.Message);
}

That's probably riddled with syntax errors, but should give you enough of an outline to get started.

Also, you really should wrap the entire operation (two INSERT s and a SELECT ) in a transaction, but I leave that as an exercise for the reader.