Introduction
A foreign key is a column or group of columns in a relational database table that links data in two tables. It is used to identify a row in one table that corresponds to a row in another table. Foreign keys can enforce data integrity, prevent duplicate data, and ensure that data is referenced correctly.
What are Foreign Keys?
Foreign keys are used to represent the relationships between tables in a database. They are often used to represent the primary key of one table in another table. In many cases, foreign keys are also used to represent the relationships between tables in a one-to-many relationship.
How are Foreign Keys Represented in ERD?
Foreign keys are an important part of any database, and they are represented in ERD by a line connecting the two tables that they are linking. The line will have a small key icon at the end to indicate that it is a foreign key.
One-to-One Relationship
Each primary key value is associated with only one foreign key value in a one-to-one relationship. For example, in a database of employees, each employee has only one employee identification number (EIN). The EIN is the primary key, and it uniquely identifies each employee. The EIN is also the foreign key in the dependents table because each dependent is associated with only one employee. In this case, the relationship between employees and their dependents is one-to-one.
One-to-Many Relationship
In a one-to-many relationship, a foreign key is used to represent the parent table’s primary key. The parent table is the table that contains the data that you want to extend to the child table. The child table is the table that contains the foreign key. In this relationship, each row in the parent table can be related to zero, one, or more rows in the child table. This is represented by a line drawn from the parent table’s primary key to the child table’s foreign key.
Many-to-Many Relationship
A many-to-many relationship is when a single entity can be linked to multiple other entities, and those other entities can also be linked to multiple other entities. In a sense, it’s like a web: each entity is connected to many other entities, and each connection can be traversed in both directions.
Consider a social network: each person (entity) can be connected to many other people, and each of those connections can be traversed in both directions. Each person has a connection (foreign key) to many other people, and each person can be the connection (foreign key) for many other people.
The main challenge with modeling many-to-many relationships is that they require an additional table, called a junction table, to store the foreign keys. The junction table is often called a Join Table or a Linking Table. In our social network example, we would need a junction table with two foreign keys: Person A and Person B.
Conclusion
In conclusion, foreign keys are a necessary part of any database design. Without them, it would be not easy to keep track of data and maintain relationships between tables. When designing your database, be sure to take foreign keys into account.