How can I insert into tables with relations?

Question

I have only done databases without relations, but now I need to do something more serious and correct.

Here is my database design:

1. Kunde = Customer
2. Vare = Product
3. Ordre = Order (Read: I want to make an order)
4. VareGruppe = ehm..type? (Read: Car, chair, closet etc.)
5. VareOrdre = Product_Orders

Here is my SQL (SQLite) schema:

CREATE TABLE Post (
    Postnr INTEGER NOT NULL PRIMARY KEY,
    Bynavn VARCHAR(50) NOT NULL
);

CREATE TABLE Kunde (
    CPR INTEGER NOT NULL PRIMARY KEY,
    Navn VARCHAR(50) NOT NULL,
    Tlf INTEGER NOT NULL,
    Adresse VARCHAR(50) NOT NULL,
    Postnr INTEGER NOT NULL 
    CONSTRAINT fk_postnr_post REFERENCES Post(Postnr)
);

CREATE TABLE Varegruppe (
    VGnr INTEGER PRIMARY KEY,
    Typenavn VARCHAR(50) NOT NULL
);


CREATE TABLE Vare (
    Vnr INTEGER PRIMARY KEY,
    Navn VARCHAR(50) NOT NULL,
    Pris DEC NOT NULL,
    Beholdning INTEGER NOT NULL,
    VGnr INTEGER NOT NULL
        CONSTRAINT fk_varegruppevgnr_vgnr REFERENCES Varegruppe(VGnr)
);

CREATE TABLE Ordre (
    Onr INTEGER PRIMARY KEY,
    CPR INTEGER NOT NULL
        CONSTRAINT fk_kundecpr_cpr REFERENCES Kunde(CPR),
    Dato DATETIME NOT NULL,
    SamletPris DEC NOT NULL
);

CREATE TABLE VareOrdre (
    VareOrdreID INTEGER PRIMARY KEY,
    Onr INTEGER NOT NULL
        CONSTRAINT fk_ordrenr_onr REFERENCES Ordre(Onr),
    Vnr INTEGER NOT NULL 
        CONSTRAINT fk_varevnr_vnr REFERENCES Vare(Vnr),
    Antal INTEGER NOT NULL
);

It should work correctly.

But I am confused about Product_Orders.

How do I create an order? For example, 2 products using SQL INSERT INTO? I can get nothing to work.

So far:

Only when I manually insert products and data into Product_Orders and then add that data to Orders = which makes it complete. Or the other way around (create an order in with 1 SQL, then manually inserting products into Product_orders - 1 SQL for each entry)

Answer 1

You should first create an order and then insert products in the table Product_Orders. This is necessary because you need an actual order with an id to associate it with the table Product_Orders.

You always should create a record in the foreign-key table before being able to create one in your current table. That way you should create a "Post", customer, type, product, order and product_order.