# Data modeling glossary

> Plain-language definitions of common data modeling terms, from attribute, cardinality and crow's foot notation to normalization, surrogate keys, junction tables, reference data, data contracts and slowly changing dimensions.

Source: https://entitymodel.com/docs/learn/data-modeling-glossary/

### Attribute

A fact recorded about an entity, such as a customer's email address. Becomes a column in a physical model.

### Cardinality

How many instances of one entity can relate to instances of another: one-to-one (1:1), one-to-many (1:N) or many-to-many (N:N).

### Chen notation

An ER diagram notation that draws entities as rectangles, relationships as diamonds and attributes as ovals. Common in textbooks.

### Composite key

A primary key made of more than one attribute, such as `order_id` plus `line_number`.

### Conceptual data model

A high-level model of the main business entities and their relationships, for agreeing on scope. See [Conceptual, logical and physical data models](/docs/learn/conceptual-logical-physical-data-models/).

### Crow's foot notation

The most common ER diagram notation, which marks the "many" end of a relationship with a three-pronged symbol. See [What is an ER diagram?](/docs/learn/what-is-an-er-diagram/)

### Data contract

A formal, machine-readable agreement between the producer and the consumers of a dataset, describing its schema, meaning and quality rules. The Open Data Contract Standard (ODCS) is one format.

### Data dictionary

A catalog of every entity and attribute with its type, meaning, rules and examples.

### DBML

Database Markup Language, a small text language for describing database schemas, used by dbdiagram.io.

### DDL

Data Definition Language: the SQL statements (`CREATE TABLE`, `ALTER TABLE`, `DROP TABLE`) that define a database's structure.

### Denormalization

Deliberately duplicating data (for example, in a star schema) to make reads simpler or faster.

### Dimensional model

An analytics design of fact tables (measurements, such as sales) surrounded by dimension tables (context, such as customer, product and date). Also called a star schema.

### Entity

A kind of thing the business keeps data about, such as Customer or Order. Becomes a table in a physical model.

### Entity-relationship diagram (ERD)

A diagram of entities, their attributes and the relationships between them.

### Foreign key

An attribute that holds the primary key of another entity, implementing a relationship.

### Forward engineering

Generating a database (SQL DDL) from a model.

### Hierarchy

A tree of records or entities in levels, such as company ownership or a product taxonomy, often modeled with a relationship from an entity to itself.

### Junction table

A table that implements a many-to-many relationship by holding the keys of both sides. Also called an associative, bridge or link table.

### Logical data model

A detailed model of entities, attributes, keys and relationships, independent of any database product.

### Lookup table

A physical table holding a reference list, such as `ref_country`.

### Natural key

An identifier that has meaning in the business, such as an email address or a tax number.

### Normalization

Organizing data so each fact is stored once, typically to third normal form (3NF), to avoid inconsistencies.

### Optionality

Whether a relationship is required on a given side: must every order have a customer?

### Physical data model

A model of tables, columns, types and constraints for one specific database.

### Primary key

The attribute (or attributes) that uniquely identify each instance of an entity.

### Reference data

Controlled lists of codes that other data points to, such as countries, currencies and statuses. See [Reference data](/docs/reference-data/).

### Reverse engineering

Building a model from an existing database or SQL script.

### Schema

The structure of a database: its tables, columns, types and constraints. Also, a named container of tables in many databases.

### Slowly changing dimension (SCD)

A technique for keeping the history of changing attributes in a dimension table, for example with valid-from and valid-to dates (type 2).

### Subject area

A group of related entities, such as Sales or Inventory, used to organize a large model.

### Surrogate key

A meaningless identifier generated by the system, such as an auto-incrementing number or a UUID.

### Third normal form (3NF)

A level of normalization in which every non-key attribute depends on the key, the whole key, and nothing but the key.