Designers' Primary Keys: When To Use Them

A primary key is a column or a combination of columns that uniquely defines each row in a table. It is a unique identifier, such as a driver's license number or telephone number. When designing a database, choosing a primary key is one of the most important steps.

There are two types of primary keys: a natural key and a surrogate key. A natural key is a key with business value that is logically related to a table, meaning that the data of a natural key exists in nature. A surrogate key, on the other hand, is a unique number generated by a database and has no business logic.

When choosing a primary key, there are several criteria to consider:

- Uniqueness: All records in a primary key must be unique.

- Simplicity: A primary key should be simple and consist of one column whenever possible.

- Stability: A primary key should be stable and not change frequently.

- Familiarity: A primary key should be meaningful to the user.

- Data type: The data type of a primary key should be numeric, an integer, or a short, fixed-width character.

- Length: The primary key should be short.

- Irreducibility: No subset of the key should uniquely identify a row in the table.

Simplicity: a primary key should be simple, using basic attributes such as whole numbers or short character strings

Simplicity is a key consideration when creating a primary key. Basic attributes such as whole numbers or short character strings are recommended to prevent confusion. This is especially important when dealing with large datasets, where simplicity can improve performance and make data retrieval, updates, and deletions faster and more efficient.

When creating a primary key, it is best to use simple, easily identifiable attributes. Whole numbers are ideal, as they are easy to generate and store. Short character strings can also be used, but it is important to avoid special characters, spaces, and differential capitalisation. For example, a driver's license number or a telephone number with an area code can serve as a simple and effective primary key.

The simplicity of a primary key is crucial for maintaining data integrity and ensuring unique identification. By using basic attributes, the risk of duplication or null values is reduced, and the primary key can serve as a reliable reference point for database operations. Simplicity also facilitates the formation of relationships between tables and improves row-level accessibility.

In addition to simplicity, other factors to consider when creating a primary key include uniqueness, stability, and relevance. It is important to ensure that the chosen primary key is unique across all records and remains stable and relevant over time. However, simplicity is a fundamental aspect that underpins the effectiveness and functionality of a primary key.

Uniqueness: a primary key must have a unique value, which means no other row in the table can have the same value in the column

Uniqueness is a fundamental characteristic of primary keys. A primary key must have a unique value, meaning no two rows in a table can have the same value in the column. This uniqueness property of primary keys ensures that each row in a table is distinct and can be uniquely identified.

The uniqueness of a primary key is essential for maintaining data integrity and preventing duplicate entries in a table. It also enables efficient data retrieval and facilitates the creation of relationships between tables through foreign keys.

When designing a database, choosing an appropriate primary key is crucial. A primary key can be a single column or a combination of columns, but it must meet the uniqueness requirement to effectively identify each row in the table.

In summary, the uniqueness of a primary key is a fundamental aspect of relational database design, ensuring data integrity, efficient data retrieval, and facilitating relationships between tables.

Stability: both the columns in the primary key and the values that make it up should not change

Stability is a key consideration when choosing a primary key. A primary key should not change because it is used to uniquely identify each row in a table. If the primary key changes, it will no longer be able to accurately identify the rows in the table, leading to data inconsistencies and errors. Therefore, it is essential to choose a primary key that is stable and unlikely to change over time.

When selecting a primary key, it is important to consider the stability of both the columns and the values that make up the primary key. The columns chosen as the primary key should be static and not subject to frequent changes. For example, using a person's name as a primary key may not be stable because people can change their names. Similarly, the values within the columns should also remain constant and unique. If the values change or duplicate, the primary key will lose its effectiveness in uniquely identifying each row.

In addition to stability, there are other factors to consider when choosing a primary key. Simplicity is important, as primary keys should be short and consist of one column whenever possible. The data type of the primary key should typically be numeric, an integer, or a short, fixed-width character. Uniqueness is also crucial, ensuring that no two rows in the table have the same value in the primary key column.

While stability is essential, it is also worth noting that primary keys can be changed if the specific needs of the users change. For example, in one application, people in a town might be identified by their driver's license numbers, while in another situation, it might be more convenient to use their telephone numbers as the primary key. However, stability should still be a key consideration to ensure the integrity and accuracy of the data.

Irreducibility: no subset of the key can uniquely identify a row in the table

Irreducibility is one of the most important design criteria for primary keys. It refers to the property that no subset of the key can uniquely identify a row in the table. In other words, if you remove any column from the primary key, it should no longer be unique. This ensures that the primary key is minimal and provides the most efficient way to uniquely identify each row in the table.

For example, consider a table with columns "FirstName", "LastName", and "Email" as the primary key. If you remove any one of these columns, the remaining columns will not uniquely identify a row. This demonstrates the irreducibility property.

Another example would be a table with "InvoiceNumber" and "ItemNumber" as a composite primary key. If you remove either "InvoiceNumber" or "ItemNumber", the remaining column will not uniquely identify an invoice or item.

By ensuring irreducibility, we create a compact and efficient primary key that minimises storage space and improves query performance. It also simplifies the database design and makes it easier to maintain and modify the database over time.

Non-null data value: the data value of a primary key should be non-null and remain constant over time

A primary key is a column in a relational database table that is distinctive for each record. It is a unique identifier, such as a driver's license number, telephone number, or vehicle identification number.

A primary key serves a unique role in the data table of a relational database management system (RDBMS). Every entity in a data model should have a primary key, which should be based on a single attribute or a group of attributes.

The data value of a primary key should be non-null and remain constant over time. This is because a non-null primary key helps to identify unique data, such as a customer ID, and prevents the duplication of records in a table. It also helps with updating or deleting only specific records and ensures that fields are not null.

For example, in a table containing customer data, a non-null primary key could be a unique customer ID that is generated for each new customer. This would ensure that each customer has a unique identifier, preventing duplicate entries and making it easier to update or delete specific customer records.

Additionally, a non-null primary key can improve the performance of database operations. For instance, when joining two tables based on a primary key, having a non-null value can speed up the process by reducing the need for additional checks or validations.

In summary, a non-null data value for a primary key is important for maintaining data integrity, uniqueness, and efficient data management in a relational database.

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