What This Topic Established

JpaRepository extends CrudRepository with pagination and batch operations — reach for it once basic CRUD isn't the whole story. A method name alone builds a real query, verified against a real H2 database; @Query takes over once a join or something else the naming convention can't express is needed.

save() on an entity that already has an id updates that row instead of inserting a new one — verified: the row count never changed.