primary-key

In databases, we need a way to uniquely identify each row/record. Without a unique identifier, we can’t reliably update, delete, or reference specific records. Multiple records might have identical data but represent different entities.

A primary key is a column (or set of columns) whose values uniquely identify each row in a database table. In EJB entity beans, the primary key is wrapped in a PrimaryKeyClass object that the container uses for identity management.

In relational databases:

1. Uniqueness — no two rows can have the same primary key value
2. Non-null — primary key cannot be NULL
3. Immutable — once set, primary key shouldn’t change

In EJB Entity Beans:

1. PrimaryKeyClass — wrapper class implementing Serializable and optionally hashCode/equals
2. getPrimaryKey() — method to retrieve current entity’s primary key
3. Finder methods — return primary keys to locate entities
4. Container uses PK — for pooling, identity, and relationship management

• Unique identification — each entity instance has distinct primary key
• Serializable — PrimaryKeyClass must implement java.io.Serializable
• Simple or composite — single field or multiple fields combined
• Container-managed — EJB container tracks entities by primary key

• Built from: Entity Bean — entity beans use primary keys for identity
• Built from: Entity Bean Identity — PK defines entity identity
• Builds into: Finder Methods — return primary keys
• Builds into: getPrimaryKey() — method to retrieve PK
• Builds into: Primary Key Class — wrapper class for PK
• Related: JDBC — SQL uses primary keys for WHERE clauses

• Composite keys — need custom PrimaryKeyClass with proper equals() and hashCode()
• Auto-generated keys — database can generate (e.g., AUTO_INCREMENT), must sync to bean
• Changing PK — don’t change primary key after creation; it breaks identity