Now we want to bring our design into the database.
#Sqlite foreign key cascade example how to#
How to Define the Foreign Key Constraint in SQL They allow us to place related data into multiple tables and then link them together to keep their integrity. That’s the reason why foreign keys are such a crucial part of database design. That is true however, then we wouldn’t be able to segregate the data into different categories within one table. You may notice that we could as well place all the data into one table called AirplaneFlight.
Here, instead of using the AirplaneId column for the FOREIGN KEY constraint, we have decided to use the AirplaneBrand and AirplaneModel columns, as they also uniquely identify each row of the Airplane table (assuming that our airline owns only one airplane of each brand-model pair listed in the table). It is also possible to use multiple columns to create such relations between tables.
The above example shows a relation between the Airplane and Flight tables using only one column. The FOREIGN KEY constraint offers various options to implement these updates or deletions, which will be discussed later on.Īnd one more important thing.
#Sqlite foreign key cascade example update#
Hence, any update or delete actions on the rows in the primary table Airplane must be reflected accordingly by the foreign table Flight. The values contained in the Flight.AirplaneId column directly refer to the values contained in the Airplane.AirplaneId column. Please note that the AirplaneId column of the foreign table Flightdoes not need to contain all the values stored in the Airplane.AirplaneId. That is how the relation is created between these tables – the AirplaneId column of the Flight table defines which airplane is used for each flight. The AirplaneId column, which is a primary key column for the Airplane table, is used as a foreign key column in the Flight table. Here, we’ve got the Airplane table (the primary table) and the Flight table (the foreign table). I believe some visual aids might be helpful here. If you need to refresh your knowledge of primary keys in SQL, I recommend reading the article What Is a Primary Key?. The foreign table references one or more columns (the primary key, which can be one or more columns) in the primary table that is how the link is created. To understand the concept of the FOREIGN KEY constraint in SQL, you can think of it as a reference link between tables that are known as the primary (or parent) and foreign (or child) tables. At the end, we’ll discuss the cardinality options that can be implemented with the FOREIGN KEY.There are various options offered by the FOREIGN KEY constraint in such situations – I’ll clarify them by including some examples. We’ll talk about what happens when the primary table column values get deleted or changed.We’ll learn about the relationship between the primary table (which provides its primary key column(s) values to the foreign table) and the foreign table (which uses the column(s) provided by the primary table as its foreign key).Next, we’ll talk about its benefits and features. I’ll explain how to define it using the CREATE TABLE statement and we’ll go through some examples. In this article, we’ll learn what the FOREIGN KEY constraint does in SQL. They are created using the FOREIGN KEY constraint on table columns. These links, called references, essentially act as connections between tables. One of relational databases’ key features is the ability to link data stored in different tables. What is a foreign key and why is it so important in relational databases? Learn all the details about foreign keys in this article.
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