> For the complete documentation index, see [llms.txt](https://courses.parottasalna.com/database-engineering/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://courses.parottasalna.com/database-engineering/entity-relationship-diagram/attributes.md).
# Attributes
Attributes are the properties or characteristics of an entity in a database. They represent the data stored about the entity and are implemented as columns in a table. Graphically represented as ovals or circles.
Here are examples of different types of attributes with explanations,
### **1. Simple Attribute**
* **Definition:** Cannot be divided into smaller parts.
* **Example:**
* **Entity:** `Student`
* **Attributes:** `StudentID`, `Name`, `Age`
* Explanation: `StudentID` is a simple, indivisible identifier for a student.
**Table Example**
```sql
CREATE TABLE Student (
StudentID SERIAL PRIMARY KEY,
Name TEXT NOT NULL,
Age INT
);
```
### **2. Composite Attribute**
* **Definition:** Can be divided into smaller sub-parts, each with independent meaning.
* **Example:**
* **Entity:** `Student`
* **Attribute:** `FullName` (composed of `FirstName` and `LastName`)
* Explanation: `FullName` can be divided into `FirstName` and `LastName`.
**Table Example**
```sql
CREATE TABLE Student (
StudentID SERIAL PRIMARY KEY,
FirstName TEXT NOT NULL,
LastName TEXT NOT NULL
);
```
### **3. Derived Attribute**
* **Definition:** Can be calculated from other attributes.
* **Example:**
* **Entity:** `Employee`
* **Attributes:** `DateOfBirth`, `Age`
* Explanation: `Age` is derived from `DateOfBirth`.
**Table Example**
```sql
CREATE TABLE Employee (
EmployeeID SERIAL PRIMARY KEY,
Name TEXT NOT NULL,
DateOfBirth DATE
);
-- Age can be calculated as:
-- SELECT DATE_PART('year', AGE(DateOfBirth)) AS Age FROM Employee;
```
### **4. Multivalued Attribute**
* **Definition:** Can have more than one value for a single entity.
* **Example:**
* **Entity:** `Student`
* **Attribute:** `PhoneNumbers` (a student may have multiple phone numbers)
* Explanation: A student can have a home phone and a mobile phone.
**Table Example**
To handle multivalued attributes, we use a separate table
```sql
CREATE TABLE Student (
StudentID SERIAL PRIMARY KEY,
Name TEXT NOT NULL
);
CREATE TABLE StudentPhone (
StudentID INT REFERENCES Student(StudentID),
PhoneNumber TEXT NOT NULL
);
```
### **5. Key Attribute**
* **Definition:** Uniquely identifies an entity in a table.
* **Example:**
* **Entity:** `Student`
* **Attribute:** `StudentID`
* Explanation: `StudentID` is a unique identifier for each student.
**Table Example**
```sql
CREATE TABLE Student (
StudentID SERIAL PRIMARY KEY,
Name TEXT NOT NULL
);
```
### Summary
| **Type** | **Description** | **Example** |
| -------- | --------------- | ----------- |
| **Simple** | Cannot be divided further. | `Name`, `Age` |
| ---------- | -------------------------- | ------------- |
| **Composite** | Can be divided into sub-parts. | `FullName` → `FirstName`, `LastName` |
| ------------- | ------------------------------ | ------------------------------------ |
| **Derived** | Calculated from other attributes. | `Age` from `DateOfBirth` |
| ----------- | --------------------------------- | ------------------------ |
| **Multivalued** | Can have multiple values for one entity. | `PhoneNumbers` |
| --------------- | ---------------------------------------- | -------------- |
| **Key** | Uniquely identifies an entity. | `StudentID` |
| ------- | ------------------------------ | ----------- |
| **Foreign Key** | References the primary key of another entity. | `StudentID` in `Enrollment` table |
| --------------- | --------------------------------------------- | --------------------------------- |