---
title: "ER diagram: employee, skill, machine"
subject: "Database Management System"
chapter: 2
key: "semester_6/database_management_system:er-employee-skill-machine"
source: https://www.bctengineer.com/q/semester_6/database_management_system/er-employee-skill-machine
site: BCT Engineering
---

# ER diagram: employee, skill, machine

Database Management System · Chapter 2: Data Models

- Asked in: 2070 Bhadra
- 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-employee-skill-machine
- 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: employee possess skill (M:1); employee operates machine_type (M:1); employee maintains machine_type; skill is needed to operate machine_type (1:1).]

## Short answer

Entities: Employee (emp_id, name, address), Skill (skill_id, skill_name), Machine_type (type_id, type_name). Relationships: Employee possess Skill (M:1), Employee operates Machine_type (M:1), Employee maintains Machine_type, Skill needed to operate Machine_type (1:1).

## Answer

The question gives the entities (employee, skill, machine_type) and four relationships with their cardinalities. The attributes below are assumed.

#### Entities and attributes
- **Employee** (emp_id, name, address): primary key emp_id.
- **Skill** (skill_id, skill_name).
- **Machine_type** (type_id, type_name).

#### Relationships
| Relationship | Between | Cardinality |
|---|---|---|
| possess | Employee - Skill | M : 1 |
| operates | Employee - Machine_type | M : 1 |
| maintains | Employee - Machine_type | (as drawn in the solution, no ratio shown) |
| is needed to operate | Skill - Machine_type | 1 : 1 |

#### ER diagram
```text
              M                    1
 [Employee]=======<possess>=======[Skill]
   |   |                              |
   |   | M                            | 1
   |   +----<operates>---+            |
   |                     | 1     <needed_to_operate>
   |                     |            |
   |    <maintains>      |            | 1
   +---------------------+---[Machine_type]
```

Here the rule "skill is needed to operate machine_type" is 1 : 1: one skill is required for exactly one machine type. Employee operates one machine type (M : 1), so many employees may operate the same type. Maintains is a separate relationship between the same two entities, so it needs its own diamond.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Employee operates Machine_type and also maintains Machine_type. How should these be shown?
   A. As one diamond, since the entities are the same
   B. As one relationship with two cardinalities
   C. Maintains is shown as an attribute of Employee
   D. As two separate relationships (two diamonds) between the same entities
   Answer: D. Operates and maintains have different meanings, so each needs its own diamond. (Key point: Separate maintains relationship)

2. What is the cardinality of 'Employee operates Machine_type'?
   A. 1 : 1, one employee per machine type
   B. M : 1, many employees may operate the same machine type
   C. 1 : M, one employee operates many types
   D. M : N, many to many
   Answer: B. As drawn, an employee operates one machine type while many employees may share a type. (Key point: Operates cardinality)

3. 'Skill is needed to operate Machine_type' is 1 : 1. What does that mean?
   A. Many skills are needed for each machine type
   B. One skill is needed for many machine types
   C. Each employee has exactly one skill
   D. One skill is required for exactly one machine type
   Answer: D. 1 : 1 pairs one skill with one machine type. (Key point: Skill needed relationship)

4. What is the cardinality of 'Employee possess Skill'?
   A. 1 : M, one employee has many skills
   B. M : 1, many employees can have the same skill
   C. 1 : 1
   D. M : N
   Answer: B. The solution shows M : 1 between Employee and Skill. (Key point: Possess cardinality)

## Related in Database Management System

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