--- title: "ER diagram: product and components" subject: "Database Management System" chapter: 2 key: "semester_6/database_management_system:er-product-component" source: https://www.bctengineer.com/q/semester_6/database_management_system/er-product-component site: BCT Engineering --- # ER diagram: product and components Database Management System · Chapter 2: Data Models - Asked in: 2070 Magh - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/database_management_system/er-product-component - Whole chapter: https://www.bctengineer.com/md/semester_6/database_management_system/chapter-2 ## Question Draw an ER diagram for the following. [The question text is not printed; the page gives only the label and the solution diagram: Product (p_no, desc, cost) is comprised of Component (c_no, desc, measure) in a many-to-many relationship; Component has a recursive relationship 'make' with attribute no.] ## Short answer Entities: Product (p_no, desc, cost), Component (c_no, desc, measure). Relationships: Product comprised_of Component (M:N), Component has recursive make (assembly, part) with attribute no. ## Answer The diagram contains two entities, a many-to-many relationship, and a recursive relationship on Component. #### Entities and attributes - **Product** (**p_no**, desc, cost). - **Component** (**c_no**, desc, measure). #### Relationships 1. **comprised_of** between Product and Component: M : N. A product is made of many components, and one component can be used in many products. 2. **make** on Component (recursive, unary): a component is made from other components. The relationship has the attribute **no** (how many units of the sub-component are used). Roles: *assembly* and *part*. #### ER diagram ```text (p_no)(desc)(cost) \ | / [ Product ] | M | N [Component]----(c_no) | | | \ | | | (desc)(measure) | | +---------+ | | | 1 +-------+ (no) (assembly) N (part) ``` The recursive line leaves Component and returns to it, labelled with the roles. In the relational schema this becomes: Product(p_no, desc, cost), Component(c_no, desc, measure), Comprised_of(p_no, c_no), Make(assembly_c_no, part_c_no, no). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is 'comprised_of' between Product and Component M : N? A. Each component belongs to exactly one product B. Each product has exactly one component C. A product has many components and a component can be used in many products D. Because Component has a recursive relationship Answer: C. Both sides can have many, so the relationship is many-to-many. (Key point: Product-Component cardinality) 2. What does the recursive relationship 'make' on Component mean? A. A component is made from other components, with roles assembly and part B. A component is made from products C. Two different entities are linked D. It duplicates the Component entity Answer: A. A recursive (unary) relationship links an entity to itself using roles. (Key point: Recursive relationship) 3. What does the attribute 'no' on 'make' record? A. The component number c_no B. The number of products using the component C. How many units of the sub-component are used D. The primary key of Product Answer: C. no is a relationship attribute giving the quantity of part used in the assembly. (Key point: Attribute no) 4. How is 'make' mapped in the relational schema? A. Make(assembly_c_no, part_c_no, no) B. Make(p_no, c_no) C. An extra column added to Product D. It is dropped since it is recursive Answer: A. The recursive relation becomes a table with two component keys plus attribute no. 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