Primary Key vs. Unique Key: What's the Difference and When Should I Use Each?

Comparison of primary key and unique key

In relational databases, keys play a vital role in maintaining data integrity and efficient data retrieval. Primary keys and unique keys are the two most commonly used key types. Understanding the differences between them is critical to optimal database design and data management.

Primary Key

The primary key is a unique identifier for each row in the table. It ensures that no two rows in the table have the same primary key field value. This is critical to maintaining data integrity as it allows the database system to quickly and accurately identify and retrieve specific records. Additionally, primary keys are often used in relational databases to enforce relationships between tables.

Unique key

A unique key simply means that the value in the specified field must be unique within the table. Unlike primary keys, unique keys allow multiple null values ​​in the table, but the values ​​must be unique in each row. This constraint is typically used to prevent duplicate entries in a column or to ensure the uniqueness of a data element such as an email address or serial number.

Main differences

• Number of occurrences: A table can only have one primary key, but can have multiple unique keys.
• Null values: Primary keys generally cannot contain null values, while unique keys can.
• Candidate Keys: Unique keys can be candidate keys, meaning they can become potential candidates for a primary key if the original primary key is compromised.
• Reference other tables: Primary keys are usually used to establish relationships between tables, while unique keys are mainly used within a single table.

When to use which key

• Primary Key: When you need a unique and non-null identifier for each row, when you need to enforce data integrity, and when you plan to use the key to link to other tables, Please use primary key.
• Unique Keys: When you want to ensure that a specific field contains unique values ​​but don't necessarily need all values ​​to be non-null, when you want to define potential candidate keys, and when you don't intend to use the key When used for relationships between tables, use unique keys.

In short, both primary keys and unique keys play an important role in database design. Understanding the differences between them is critical to optimizing data storage and retrieval and maintaining data integrity.

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