Java使用外键将数据插入数据库

Question

So I am creating a small application for a football organisation that needs to be able to add teams to the database.

My database has the following ERD:

I have the following code to add teams to my database:

public void toevoegenPloeg(Ploeg ploeg) throws DBException, ApplicationException {
    //connectie tot stand brengen
    System.out.println(ploeg.getTrainerID());
    try (Connection connection = ConnectionManager.getConnection();) {
        //statement opstellen
        try (PreparedStatement statement = connection.prepareStatement("insert into ploeg (naam, niveau, trainer_id) values(?,?,?)");) {
            statement.setString(1, ploeg.getNaam());
            statement.setString(2, ploeg.getNiveau());
            statement.setInt(3, ploeg.getTrainerID());
            statement.execute();


        } catch (SQLException ex) {
            throw new DBException("SQL-exception in de toevoegenPloeg-methode - statement" + ex);
        }
    } catch (SQLException ex) {
        throw new DBException("SQL-exception in de toevoegenPloeg-methode - connectie " + ex);
    }

}

It has to be possible to add teams without a trainer. Like this:

PloegTrans PT = new PloegTrans();
PersoonTrans PeT = new PersoonTrans();

Ploeg ploeg1 = new Ploeg();
ploeg1.setNiveau("U9");
PT.ploegToevoegen(ploeg1);

Trainer_id is an int and because I haven't defined the trainer_id. The trainer_id becomes the default of an int, 0.

But then I get a Foreign Key exception, because the database looks for a trainer with id 0.

How can I overcome this?

How can I initialize my int as a "null"?

Answer 1

Use statement.setNull(3, java.sql.Types.INTEGER); or construct the statement as insert into ploeg (naam, niveau) values(?,?) , so trainer_id will default to NULL .

Answer 2

On the POJO's side, the trainer ID should be a java.lang.Integer , not a primitive int , in order to allow null s. On the JDBC side, you could use setObject instead of setInt , which does accept null s:

// getTrainerID() returns either an Integer instance or null
statement.setObject(3, ploeg.getTrainerID());