![]() We first create the country table with a primary key In the code below, we drop and re-create both tables – this time, defining the primary and foreign keys: Before we can create a foreign key on the table city, we need a primary key or a unique constraint on the country table. A foreign key constraint will inform the database about the relation between the tables.Ī foreign key constraint is a database constraint that binds two tables. something that points to a non-existent record in the country table). Moreover, the database will allow a user to put an invalid value in untry_id (e.g. However, the database doesn't know that both tables are related. Using the untry_id value, we can join the city table with country and find the record associated with any city. The value in the column untry_id should refer to a valid country_id in the country table. Notice that city has a column called country_id that is the link between both tables. We can say that these tables are related.īelow, we can see the SQL to create the country and city tables: ![]() Thus, part of the city information – the name of the country where the city is located – is actually stored in the country table. ![]() For this last attribute, we add a column called country_id that references a record in the country table. Initially, we think of creating a single table called city to store the city name, a numeric city_id, and the country where this city is located. We also want to represent the world’s major cities in this database. Suppose we have a database with country data stored in a table called country. In that case, we use a foreign key to link both tables in the database. When designing a database, it often happens that an element can’t be fully represented in just one table. All SQL developers need to know what SQL foreign keys are, how they work, what data values are allowed in them, and how they’re created. Usually they work with primary keys to connect two database tables, like a virtual bridge. I just tested these SQLite foreign key examples on my system, using SQLite version 3.4.0, and they all work fine.What does a foreign key do in SQL? Why would you need one? Learn the ABCs of foreign keys in five minutes.įoreign keys are a central concept in SQL databases they allow us to enforce data consistency. INSERT INTO customers VALUES (null, 'FOOBAR', '200 Foo Way', 'Louisville', 'KY', '40207') INSERT INTO customers VALUES (null, 'ACME, INC.', '101 Main Street', 'Anchorage', 'AK', '99501') INSERT INTO salespeople VALUES (null, 'Barney', 'Rubble', 10.0) INSERT INTO salespeople VALUES (null, 'Fred', 'Flinstone', 10.0) If you'd like to test this SQLite foreign key example in your own SQLite database, here's some sample data for each of these tables: Next, define a SQLite table that has two foreign keys, one that relates a new orders table back to the customers table, and a second foreign key that relates the orders table back to the salespeople table:įOREIGN KEY(customer_id) REFERENCES customers(id),įOREIGN KEY(salesperson_id) REFERENCES salespeople(id)Īs you can see, the SQLite foreign key syntax is very similar to other databases. To show how this works, first define two database tables that don’t have any foreign keys: Here’s a quick SQLite foreign key example. ![]() The SQLite database does support foreign keys, and its foreign key syntax is similar to other databases. SQLite foreign keys FAQ: Can you show me how to define foreign keys in a SQLite database table design?
0 Comments
Leave a Reply. |