Foreign keys are columns in a table that establish relationships with values in another table. They ensure database-level relationships, with the table containing the foreign key being the “child” and the other table being the “parent”. A good understanding of primary keys is necessary for working with foreign keys.
Databases are made up of tables where data or information is stored. A foreign key is a mechanism in a relational database that allows such tables to be associated or for the tables to have a relationship with each other. There are unique features that a foreign key must have. Must be a column or group of columns in a table whose values establish a relationship with values in another table within the same database.
A foreign key ensures that rows in one table match rows in another table, thereby establishing database-level relationships or references. The table that contains the foreign key is the “child” and the other table is the “parent”. It is possible for the value of a foreign key to be null or empty and actually refer to or refer to what is known as the primary key of the table in which it is located. This is known as self referentiality.
You need to have a good understanding of what a primary key is before attempting to fully understand the concept behind foreign keys. For example, if you have two tables in a database that store information about published books for a website that sells books, one table might be the publishers table and the other the books table. The publishers table can consist of two columns, a primary key that is always unique for each record, and a publisher name column. A minimum of three columns would make up the books table, containing the primary key or unique identifier for each book, the book titles, and a column through which it would relate to the publishers table. That column would be the foreign key.
The primary key for the publishers table could be something like “pub_id” with values of: P01, P02, P03, etc. A relationship would be established with the books table if it contained a pub_id column with the same values. This would be the foreign key that would relate this child table to its parent, the publishers table.
A foreign key can, however, have a different column name than the parent key it references. A good understanding of how primary and foreign keys work is essential to maintaining referential integrity. Very large databases that are composed of many tables or involve a junction table have more than one type of relationship, which can complicate the task of working with foreign keys.
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