Inserting data with foreign keys and avoiding duplication

Question

I'm very beginner at MySQL and have just started to play around with foreign keys and INNER JOIN operator. So I've made a few tables like:

CREATE TABLE `models` 
(
`id` TINYINT UNSIGNED NOT NULL AUTO_INCREMENT, 
`name` VARCHAR (255) NOT NULL, 
`price` MEDIUMINT UNSIGNED NOT NULL,
PRIMARY KEY( `id` )
);

CREATE TABLE `vendors` 
(
`id` TINYINT UNSIGNED NOT NULL AUTO_INCREMENT,  
`name` VARCHAR (255) NOT NULL,
`id_model` TINYINT UNSIGNED NOT NULL,
PRIMARY KEY( `id` ),
FOREIGN KEY (`id_model`) REFERENCES models(`id`)
);

CREATE TABLE `cars` 
(
`serial_number` MEDIUMINT UNSIGNED NOT NULL,  
`id_vendor` TINYINT UNSIGNED NOT NULL,
FOREIGN KEY (`id_vendor`) REFERENCES vendors(`id`),
PRIMARY KEY( `serial_number` )
);

I know how to get output with that. However, the problem is that I don't know how to insert data properly. All I can do is insert data table by table. But how to do it in one query, and if I am inserting Honda Civic and already have Honda Accord, for example, wouldn't it duplicate Honda vendor in the database?

Answer 1

You do need to check if duplicated record exists yourself.

IF EXISTS (SELECT * FROM vendors WHERE Name = 'Honda')
BEGIN
    -- Insert into cars with existing vendor id
END
ELSE
BEGIN
    IF EXISTIS (SELECT * FROM models WHERE Name = 'your model name')
    BEGIN
        -- insert into vendors with existing model id
    END
    ELSE
    BEGIN
        -- insert into models
        -- insert into vendors
        -- insert into cars
    END
END

You can create stored procedure for it and pass car, vendor and model as parameters. Or you can list models first, insert them; then all vendors and all cars. Just a silly answer. Welcome more sophisticated solutions.

Answer 2

It seems that the structure of the database is not really coherent. Maybe I don't understand what you are trying to do, but ... anyway, here goes.

Assuming that what you want to do is store a list a of cars in a properly normalized relational database, first thing you want to do is think what is happenning in "real life":

• Manufacturers (vendors) make/sell cars
• Manufacturers (vendors) make different models of cars
• Cars have a model (and a serial number in your case)
• Models belong to a vendor (manufacturers)

Considering this, your table structure is:

Vendors
- id
- name

Models
 - id
 - name
 - vendor ( foreign key => vendor.id )

Cars    
- id
- serial_number
- model ( foreign key => model.id )

You don't need to have a reference to the vendor in the cars table becoause you have a reference to the model, which in turn has a reference to the vendor.

Whe inserting, you do it one by one, making sure that the foreign key entries already exist.

• When you insert a car object, you just need to provide the model id.
• When you insert a model object you need to provide a vendor id.

So the Honda Civic/Accord situation does not duplicate Honda. The Tables should be something like this:

Vendor

id, name
1, "Honda" 

Model

id, name, vendor
1, "Civic", 1
2, "Accord", 1

Cars

id, serial_no, model
1, "A serial", 2 -> a honda accord
2, "Another serial", 1 -> a honda civic

Hope this helps somewhat.