add a writeup on master-part relationships

Author: dimitri-yatsenko

book/30-database-design/050-relationships.ipynb

@@ -16,6 +16,8 @@
     "\n",
     "Referential constraints establish relationships between tables and are enforced by [foreign keys](030-foreign-keys.ipynb). They ensure that references between tables remain valid and prevent orphaned records.\n",
     "\n",
+    "In DataJoint, foreign keys also participate in the **relational workflow model** introduced earlier: each dependency not only enforces referential integrity but also prescribes the order of operations in a workflow. When table `B` references table `A`, `A` must be populated before `B`, and deleting from `A` cascades through all dependent workflow steps. The resulting schema is a directed acyclic graph (DAG) whose arrows describe both data relationships and workflow execution order (see [Relational Workflows](../20-concepts/04-workflows.md)).\n",
+    "\n",
     "## Foreign Keys Establish 1:N or 1:1 Relationships\n",
     "\n",
     "When a child table defines a foreign key constraint to a parent table, it creates a relationship between the entities in the parent and child tables. The cardinality of this relationship is always **1 on the parent side**: each entry in the child table must refer to a single entity in the parent table.\n",
@@ -1253,7 +1255,7 @@
    "source": [
     "## Sequences\n",
     "\n",
-    "DataJoint schemas are directional: dependencies form a *directed-acyclic graph* (DAG) representing sequences of steps or operations.\n",
+    "As discussed in the [Relational Workflows](../20-concepts/04-workflows.md) chapter, DataJoint schemas are directional: dependencies form a *directed-acyclic graph* (DAG) representing sequences of steps or operations.\n",
     "The diagrams are plotted with all the dependencies pointing in the same direction (top-to-bottom or left-to-right), so that a schema diagram can be understood as an operational workflow.\n",
     "\n",
     "Let's model a simple sequence of operations such as placing an order, shipping, and delivery.\n",