What is the difference between a primary key and a unique constraint?

Primary Key vs. Unique Constraint: In-depth explanation

Primary keys and unique constraints form the structural integrity in any database design. Primary keys are the linchpins, featuring prominently in the establishment of foreign key relationships and swiftly retrieving data. By default, they are often allied with clustered indexes, dictating the physical storage of data in the database.

On the other hand, unique constraints enforce uniqueness across other columns where null values are permissible and symbolic of absent data. For instance, when we need to ensure the uniqueness of an attribute other than the primary key, such as email, we resort to unique constraints.

These key concepts affect data integrity and usability. Remember, thoughtlessly defining these keys could invite data discrepancies and clutter, thus necessitating a clear understanding of both.

Cherry-picking between Primary Key and Unique Constraint

Designing a database necessitates careful selection of primary keys to represent the essence of the entity enclosed within each table. Often, we inappropriately label candidate keys as primary keys without considering their future scalability and normalization needs, thereby causing a ripple effect that may necessitate a costly restructuring of the database.

Contrarily, unique constraints are our go-to when we need to enforce uniqueness on additional columns that don't qualify as primary keys. For instance, for polymorphic relationships, where multiple types of related records are stored in one place, unique constraints are the right choice.

Visualizing Primary Key vs. Unique Constraint

Stumbling upon complex database concepts? This comparison with everyday objects will help:

| Aspect             | Passport              | Driver's License                      |
| ------------------ | --------------------- | ------------------------------------- |
| Purpose            | 🌐 Unique Identity    | 👤 Unique Permission                 |
| Mandate            | 🛂 Always Required    | 🚗 Optional                         |
| Uniqueness         | 🆔 Globally Unique    | 🔢 Unique per State                  |
| Nullability        | ❌ No Nulls Allowed   | ❓ Nulls Possible                     |

Primary Key (Passport): Like your passport, a unique, mandatory identifier, recognized worldwide.

Unique Constraint (Driver's License): Like your driver’s license, unique on a state level and optional.

Tweaking performance with indexing strategies

The relationship between primary keys, unique constraints, and indexing is crucial to performance tuning. Primary keys often come with clustered indexes, and while enhancing read operations, it may affect intense write operations.

However, non-clustered indexes that come with unique constraints can maintain quick lookup speeds with lesser effect on insert/update operations. Decisions around primary keys or unique constraints should therefore take current and future data access patterns into account.

Promoting data integrity

In the world of databases, primaries keys are renowned for their implication of referential integrity, enhancing both foreign key constraints' reliability. Unique constraints, despite their inability to maintain relationships by default, require thorough inspection to uphold relationships across tables.

When nullability is used thoughtfully, primary keys provide a non-null constraint, ensuring that every record has a unique identifier. On the other hand, unique constraints welcome nulls, catering to unknown or irrelevant information. In multi-tenant applications, unique constraints keep tenant-specific uniqueness even when the domain may overlap across tenants.