Entity Mapping: Keys, Columns & Annotations

A composite primary key uses two or more columns as the PK, mapped via @EmbeddedId (key as one embedded object) or @IdClass (multiple @Id fields mirroring a separate ID class).

@ElementCollection maps a collection of values or embeddables with no identity, stored in a separate table, where any modification triggers a delete-all and reinsert of the entire collection.

IDENTITY uses database auto-increment but disables batch inserts, SEQUENCE pre-fetches IDs in blocks for batch-friendly inserts, TABLE is slow with row locking, AUTO lets the provider decide, and UUID generates IDs in the JVM with no database round-trip.

@Column customizes how a Java field maps to a database column, @Transient (JPA) excludes a field from persistence entirely, and Java's transient keyword excludes from serialization only while Hibernate still maps it.

@EmbeddedId embeds one @Embeddable key object (access via getId().getX()); @IdClass keeps separate @Id fields on the entity plus a mirror key class, and both need Serializable, equals, and hashCode.

@ElementCollection stores value objects (basics or @Embeddable) with no identity in a @CollectionTable owned by the parent; @OneToMany relates independent entities with their own id and lifecycle.

IDENTITY uses DB auto-increment (no batching); SEQUENCE pre-fetches id blocks (preferred, batchable); TABLE emulates sequences portably but slowly; AUTO lets the provider choose; UUID is app-generated and distribution-friendly.

@Column tunes name, nullability, length, precision/scale, uniqueness, and insert/update inclusion; JPA @Transient excludes from the database, Java transient excludes from serialization.