Have a foreign key reference multiple tables? - sql

Question

I have 3 tables:
Shirts(model, color)
Gloves(model, color)
Socks(model, color)
Where model is primary key in all 3 tables
Now I need to make another table:
Products (model, price)
So, in products I need to get all models from my first 3 tables, in one single column. How can I do that?

Answer 1

In my opinion, you've designed it wrong. Suggestion (as a comment under the question, saying that products should be referenced by 3 another tables) is - again, in my opinion - wrong.
You shouldn't create separate tables for shirts, gloves or socks. What will you do when you start selling hats or trousers or shoes? Will you create new tables for all of those? Of course not - those are just clothes (products) types.
So - create one table that contains all types; when new type appears, simply add a new row into that table and reference it from the products table.
Something like this:
SQL> create table product_type
2 (id_type number primary key,
3 name varchar2(30)
4 );
Table created.
SQL> create table products
2 (id_product number primary key,
3 id_type number references product_type,
4 color varchar2(20),
5 price number
6 );
Table created.
SQL> insert into product_type
2 select 1, 'shirt' from dual union all
3 select 2, 'glove' from dual union all
4 select 3, 'socks' from dual;
3 rows created.
SQL> insert into products
2 -- shirt
3 select 100, 1, 'red', 100 from dual union all
4 -- gloves
5 select 101, 2, 'blue', 150 from dual;
2 rows created.
SQL>
Here come the shoes:
SQL> insert into product_type
2 select 4, 'shoes' from dual;
1 row created.
SQL> insert into products
2 select 113, 4, 'brown', 400 from dual;
1 row created.
SQL>
Conclusion: read about normalization.
If someone says that "colors should be in a separate table", well - perhaps, that wouldn't be a bad idea either. Furthermore, does the products table have to be expanded by a date column (which would show what price was valid at certain period)? Not a bad idea either. There are numerous options you can include into the model. I just tried to point you into the right direction.

Answer 2

A foreign key can be created from the Products table to each of the other tables. However, creating a foreign key from each of the other tables (Shirts, Gloves, Socks) is not possible (unless each Product exists in all three tables). A foreign key from Shirts, Gloves, and Socks would basically say: before an insert into the Products table ensure that there is a record in each of the three other tables with the same key.
As suggested by the other posters, it is probably better to consider a slightly different design (e.g. add a type column to the products table).
Also, by convention table names should be singular (Product, Shirt, Glove, Sock).

Answer 3

If you mean in an environment like phpMyAdmin, MySQLWorkbench etc. you can just manually go to each product table 'Shirt','Gloves','Socks' and add a reference/foreign key of the field 'Model' to the the field 'Model' in the table 'Product'. The way to do this depends entirely on the programm you are using. Note that ofcourse you should create the new Table either manually or by SQL code before you do this.
There is no problem for multiple tables to point to same other table via foreign keys.
If you want to write SQL code to this you can use something like this, which is the other way around (use three foreign keys in the table 'Product', one for each other table). Note that the tables Socks,Shirts and Gloves must have already been created.
DROP TABLE IF EXISTS `Product` ;
CREATE TABLE `Product` (
`Color` varchar(8),
`Model` varchar(14),
PRIMARY KEY(`Model`),
FOREIGN KEY('Model') REFERENCES `Socks`(`Model`) ON DELETE CASCADE,
FOREIGN KEY('Model') REFERENCES `Gloves`(`Model`) ON DELETE CASCADE,
FOREIGN KEY('Model') REFERENCES `Shirts`(`Model`) ON DELETE CASCADE
) ;
The way you want is something like this (done three times one for each table). Note that as i said before the table Product must have been already created.
DROP TABLE IF EXISTS `Socks` ;
CREATE TABLE `Socks` (
`Color` varchar(8),
`Model` varchar(14),
PRIMARY KEY(`Model`),
FOREIGN KEY('Model') REFERENCES `Product`(`Model`) ON DELETE CASCADE
);