The primary key is a field or field combination in a database table that uniquely identifies each row of data. Its core function is to ensure data integrity, improve query efficiency and support foreign key constraints. The primary key must satisfy uniqueness and non-emptyness. Each table can only have one primary key, but it can consist of multiple columns to form a composite primary key. It maintains data consistency by preventing duplicate records and providing a basis for foreign key references, while automatically creating indexes to speed up queries. When selecting primary keys, the principles of stability, simplicity and business-free meaning should be followed. Common methods include self-incremental integers, GUID/UUID and natural keys if necessary. Composite primary keys are suitable for situations where a single field cannot uniquely identify data. For example, the student ID and the course ID form the primary key of the course selection schedule, but due to its complexity and performance impact, it is recommended to use it only if necessary.

In SQL database design, Primary Key plays a very core role. It is not only used to uniquely identify each row of data in a table, but also directly affects data integrity, query efficiency and association relationships between tables.

What is a primary key?

A primary key is a combination of one or more fields, and its value must be unique throughout the table and cannot be NULL. Each table can only have one primary key, but this primary key can contain multiple columns (called composite primary keys). For example, in the user table, user_id is usually used as the primary key to ensure that each user has a unique identity.

There are a few points to pay attention to when setting the primary key:

• Values ??must be unique
• NULL is not allowed
• Each table can only have one primary key

How to ensure data integrity with primary keys?

The existence of primary keys can effectively prevent the insertion of duplicate data, while providing a basis for foreign key constraints. For example, referring to the primary key user_id of the user table as a foreign key in the order table can ensure that each order corresponds to a real user. Without a primary key, this reference relationship cannot be reliably established and the database structure becomes chaotic.

Additionally, primary keys automatically create clustered indexes (by default), which helps speed up primary key-based searches and improve overall query performance.

How to choose the right primary key?

When selecting a primary key, several principles are generally followed:

• Stability : Try to avoid using fields that may be changed as primary keys, such as email or mobile phone number.
• Simplicity : The simpler the primary key, the better. It is recommended to use self-incremental integers (such as INT IDENTITY ) or UUID.
• Business-free meaning preferred : Although fields like ID numbers seem "naturally unique", they tend to carry sensitive information or business logic and are not suitable as primary keys.

Common practices include:

• Use Auto-increment ID
• Use GUID/UUID (for distributed systems)
• Use Natural Key when you really need it

When to use the compound primary key?

Sometimes a single field cannot uniquely identify a row of data, so multiple fields need to be used to form a composite primary key. For example, in the course selection record sheet, the combination of student ID and course ID is used as the primary key to ensure that a student does not repeat the same course.

However, compound primary keys also have some disadvantages:

• Query and maintenance are more complex
• Foreign key references must also fully match fields of composite primary keys
• Larger index size affects performance

Therefore, unless there is a clear requirement, it is recommended to prioritize single-column primary keys.

Basically that's it. Primary keys are one of the most basic and critical parts of database design. Choosing well can make the entire system more stable and efficient.

The above is the detailed content of Explain the Role of Primary Keys in SQL Database Design.. For more information, please follow other related articles on the PHP Chinese website!