The Keys & Relationships of Relational Databases

This is a continuation of my posts on relational databases, started here. Previous posts on this theme, “Designing a Relational Database – Data Modeling, “Let’s Talk About Database Schema“, and “The Exasperating Topic of Database Indexes“.

In the context of relational databases, “keys” and “relationships” are fundamental concepts that help in ensuring data integrity, organizing data efficiently, and enabling meaningful data retrieval. Let’s break these down:

1. Keys

A key is an attribute (or a set of attributes) that uniquely identifies a record in a table. Various types of keys exist in relational databases:

  • Primary Key: This uniquely identifies each record in a table. A primary key cannot have NULL values, and each value must be unique. A table can have only one primary key, which may consist of single or multiple columns.
  • Foreign Key: This is an attribute or set of attributes in a table that refers to the primary key in another table. It establishes a relationship between two tables. A foreign key ensures records in one table correspond to records in another.
  • Unique Key: Similar to a primary key, it uniquely identifies each record in the table. The difference is that a table can have more than one unique key, but only one primary key. Unique keys can have NULL values, but the values must be unique.
  • Composite Key: When more than one attribute is used to uniquely identify a record in the table, such a key is called a composite key.
  • Candidate Key: Any attribute or set of attributes that could serve as the primary key is a candidate key. It means it can uniquely identify records in the table.
  • Super Key: It is a set of attributes that, when taken collectively, can be used to uniquely identify records. A super key can have additional attributes that are not strictly necessary for unique identification.

2. Relationships

Relationships in relational databases determine how tables connect to one another and how data correlates:

  • One-to-One (1:1) Relationship: In this relationship, one record in a table is related to one and only one record in another table. For instance, each employee in a company might have one unique work ID.
  • One-to-Many (1:N) or Many-to-One (N:1) Relationship: One record in a table can be related to one or more records in another table. For example, one customer can place many orders, but each order is made by one customer.
  • Many-to-Many (M:N) Relationship: Multiple records in one table are related to multiple records in another table. For instance, students and courses. One student can enroll in many courses, and one course can have many students. This relationship is typically resolved in relational databases using a junction table (or bridge table).

Relationships are typically enforced using Foreign Keys. When a foreign key in one table refers to the primary key of another, it creates a linkage that ensures the validity of the data and maintains referential integrity.

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