45 lines
1.3 KiB
Markdown
45 lines
1.3 KiB
Markdown
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---
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tags:
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- Programming_Languages
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- Databases
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- sql
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---
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# SQL: Creating relationships between tables with `PRIMARY` and `FOREIGN` keys
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We will demonstrate with an example. 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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The `sales` table:
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````
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sale_id model_id sale_date employee_id price
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---------- ---------- ---------- ----------- ----------
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1 44 2020-07-27 tbishop 399.99
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2 22 2021-02-07 tbishop 200.99
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````
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Creating the `returns` table and establishing relationship with `sales` using the `FOREIGN KEY` keyword:
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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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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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);
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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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