Entity-relationship model facts for kids

A sample ER diagram showing how different parts of data connect.

In software engineering, an Entity-relationship model is a way to plan how information is stored in a computer. Think of it as a blueprint for a database. People often call it an ER model for short.

ER models help us design relational databases. They show how different pieces of information relate to each other. For example, they can show how a student's name connects to their classes and grades.

A person named Peter Chen first suggested using this way of modeling data in 1976. Other people had similar ideas before him, and the ER model has changed and improved since then.

Understanding ER Models

Two entities, like "Student" and "Course," connected by a relationship.

An entity, like "Student," with details called attributes, such as "Name" and "Age."

A relationship, like "Enrolls In," can also have details, such as "Enrollment Date."

A Primary key is a unique ID, like a student's ID number, that helps find their information.

In an ER model, there are three main parts that help us organize data:

• An entity is like a real-world thing or concept. It could be a person, a place, an object, or an event. For example, a "Student" is an entity, or a "Book" is an entity.
• A relationship shows how two or more entities are connected. For instance, a "Student" might "Enroll In" a "Course." "Enroll In" is the relationship.
• Both entities and relationships can have attributes. Attributes are details or facts about an entity or a relationship. For a "Student" entity, attributes could be "Name," "Age," or "Student ID." For the "Enroll In" relationship, an attribute might be "Enrollment Date."

To find a specific entity or relationship in a database, we need a special attribute or a set of attributes that is unique. This unique identifier is called a primary key. It's like a unique ID number for each item.

After mapping out all the connections, we also add something called cardinality. Cardinality tells us how many entities are involved in a relationship. For example, one student can enroll in many courses, or one course can have many students. This information is important when building the actual database.

Building a Database with ER Models

Once an ER model is complete, it makes creating a relational database much easier. Here's how it works:

• Every type of entity in your ER model becomes a "table" in the database. A table is like a spreadsheet that holds specific information.
• Every attribute of an entity becomes a "column" in that database table. For example, if you have a "Student" table, you might have columns for "Name," "Age," and "Student ID."
• Relationships also help define how tables connect, often by using primary keys to link information between different tables.

Why ER Models Are Useful (and When They Aren't)

An ER model is great for showing how different pieces of information relate to each other. It's especially good for designing databases that store structured data, like lists of customers, products, or students.

However, ER models are not always the best choice for every type of data. For example, if you have very unstructured data, like a collection of random notes or images, an ER model might not fit well.

Also, ER modeling is usually used when you are designing a brand new database. It can be harder to use ER models to describe or change a database that already exists.

Even though ER models are powerful, they are not always used as a separate step in designing databases anymore. This is because many modern tools for databases can create diagrams that look very similar to ER diagrams directly from existing databases. These tools also offer different ways to view the information.

See also

In Spanish: Modelo entidad-relación para niños