81 lines
3.1 KiB
Markdown
81 lines
3.1 KiB
Markdown
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---
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categories:
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- Databases
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- Programming Languages
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tags: [SQL, relational-database]
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---
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# Creating views with foreign keys
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We utilise **foreign** and [primary keys](/Databases/Relational_Databases/Primary_key.md) to create relationships between tables in SQL. Foreign keys link data in one table to the data in another table and are how we cross-reference data in SQL.
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<!-- We can use this technique to create new tables from existing tables in the database or to generate [views](/Databases/Relational_Databases/Views_in_relational_databases.md). -->
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In essence you use the primary key of one table to access data in another table.
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Let's say we have a `sales` table:
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| saleId | modelId | saleDate | employeeId | price |
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| ------ | ------- | ---------- | ---------- | ------ |
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| 1 | 44 | 2020-07-27 | tbishop | 399.99 |
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| 2 | 22 | 2021-02-05 | tbishop | 200.99 |
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In SQL this would be set up as follows:
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```sql
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CREATE TABLE sales (
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saleId integer PRIMARY KEY,
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modelId integer,
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saleDate date,
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employeeId text,
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price float
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)
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```
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For every model that is sold it is possible for the customer to return it. This data will be kept in another table `returns`. Every model sold will have an entry in `sales` but may or may not have an entry in `returns` (not every customer will return an item).
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We want to establish a relationship between the two tables so that if an item is returned we can trace it back to its original sale.
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As the `saleId` is the primary key in `sales` this means it is a unique identifier for each model sold. We will therefore use this in our `returns` table to track the sale data.
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Here's our `returns` table:
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| returnId | saleId | returnDate | reason |
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| -------- | ------ | ----------- | ---------------------------------------------- |
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| 7899 | 1 | 2020-11-218 | New device issued under warranty due to defect |
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| 6711 | 2 | 2022-09-02 | Returned gift |
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n this table `saleId` is identical to `saleId` in sales. It is the primary key in `sales` but a foreign key in `returns`. There is a one-to-one correspondance at work here. If a device has been returned it must have an entry in `returns` and the `salesId` of the entry in `returns` must match the `salesId` in `sales`.
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We already have the `sales` table. We want to create new table called `returns` that will sustain a one-to-one relationship with `sales`. We are going to use the `sale_id` as our foreign key in `returns`. This is the primary key in `sales`.
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We establish this with the following SQL:
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```sql
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CREATE TABLE returns (
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returnId integer PRIMARY KEY,
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saleId integer NOT NULL,
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returnDate date,
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reason text,
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FOREIGN KEY (sale_id) REFERENCES sales(sale_id)
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);
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```
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https://www.cockroachlabs.com/blog/what-is-a-foreign-key/
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---
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Here's an example with more than one foreign key:
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```sql
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CREATE TABLE returns (
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return_id integer PRIMARY KEY,
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sale_id integer NOT NULL,
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employee_id text NOT NULL,
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date_returned text,
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reason text,
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FOREIGN KEY(sale_id) REFERENCES sales(sale_id),
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FOREIGN KEY(employee_id) REFERENCES employee(employee_id)
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);
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```
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