Define the relationship dependencies between tasks.

Task relationship dependencies are typically expressed through foreign keys or join tables. In a relational model, a task can reference a parent task via a parent_task_id column, enforcing referential integrity so that a child cannot exist without its parent. Cascading actions (ON DELETE CASCADE, ON UPDATE CASCADE) propagate changes automatically. For many‑to‑many dependencies, a junction table (e.g., task_dependencies) holds pairs of task IDs. Business rules such as "a task cannot start until all its dependencies are complete" are enforced at the application level or via triggers. Proper indexing on the dependency columns improves query performance when retrieving task hierarchies or detecting cycles.