SharePoint uses lookup fields to represent the Parent-Child relationship. For child list items it is OK as you can trace back to its parent from the lookup field. For parent the OOB forms are not good enough as they are not displaying the children belong to it. In real world it is often required that when view the parent list item it also displays its children, further more it is great if user can add child list items directly from the parent. In SharePoint on-premise we can use visual web parts and code the parent/child logic and use web part pages to replace the parent item forms. In SharePoint online we can't do server model code and visual web parts, so the solution is almost NO CODE and just a little bit client side scripts, and they work beautifully! Here I will do a step by step demonstration to explain the ideas: Step 1: Create a parent list called "Category" and a child list called "Product", in Product list add a lookup field to reference Cat...