Primary Keys vs. Unique Indexes: When Should You Choose Which?

Key choices in database design: primary key and unique index

In the world of database design, the choice of using a primary key or a unique index is often confusing. While primary keys are widely considered fundamental, special cases often arise in real-world projects that require a different approach.

Primary key: the cornerstone of tradition

The primary key enforces uniqueness and non-nullability, ensuring that each row in the table is uniquely identified by a specific column or set of columns. This constraint plays a vital role in maintaining data integrity and preventing duplicate values.

Unique Indexes: Flexible Alternatives

On the other hand, a unique index imposes a unique constraint on a specific column, but allows null values. This flexibility is useful when null values ​​are allowed. Unlike primary keys, multiple unique indexes can coexist on a table, providing greater flexibility in data modeling.

Comparison of advantages and disadvantages: actual comparison

Primary key:

• Advantages:
  • Enforce uniqueness and non-emptiness to ensure data integrity.
  • Automatically create clustered indexes to optimize data access.
• Disadvantages:
  • The number of unique columns is limited to one.
  • Performance may be affected in tables with large row counts due to potential index bloat.

Unique index:

• Advantages:
  • Provides greater flexibility with null values.
  • Allows multiple unique constraints to enhance data modeling capabilities.
  • Minimize index bloat, potentially improving performance on large tables.
• Disadvantages:
  • itself does not enforce non-emptiness and requires additional constraints.
  • For best performance, you may need to manually create a clustered index.

Replication considerations for Microsoft SQL Server

In Microsoft SQL Server, replication strategy affects the selection of primary keys and unique indexes. Primary keys are often used as replication keys to ensure data integrity in replicated databases. However, in some cases, a unique index may be preferable to avoid conflicts and optimize performance in a replicated environment.

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