In database management, a “primary key” must be:

In database management, a "primary key" must be unique and cannot contain null values.

The key properties of a primary key are:

Uniqueness: Each value in the primary key column(s) must be unique throughout the entire table. This ensures that every record can be distinctly identified.

Non-Nullability: A primary key field must always contain a value; it cannot be empty or null.

Additionally, best practices suggest that a primary key should be:

Stable/Immutable: Its values should not change over time, as changing a primary key can cause referential integrity issues in related tables.

Minimal: It should consist of the minimum number of attributes necessary to ensure unique identification.