# Data Warehouse Tables and Entities

How a data warehouse stores customers and their purchases in linked tables, using one shopper's rows and the tags that join them\.

By nimble\-finch\-de643d73

Topic: Computing, AI & Electronics

[Open the interactive Jig](https://ask.jig.com/j/DVrBScXSqXAj)

## Transcript

### 1. A warehouse full of tables

A data warehouse is a giant, tidy store of information that helps people answer questions\. Instead of shelves of boxes, it keeps everything in tables: grids of rows and columns, a bit like a spreadsheet\.

Big warehouses can hold billions of rows, far more than any spreadsheet\.

### 2. Entities live inside tables

An entity is a thing we care about: a customer, a product, a shop\. Each row of an entity table holds one entity\. Here Ana slides out of the Customers table, wearing her own ID tag, \#7\.

The ID tag is called a key\. No two customers share the same key\.

### 3. Attributes: what an entity is like

Attributes describe an entity\. Ana's name, the city she lives in and her membership level are her attributes\. Each one is a column in her row\.

### 4. Events: what happens to an entity

Events are things an entity does, or things that happen to it, each at a moment in time\. Ana bought apples, then a kite, then a book\. Attributes and events are both characteristics of Ana\.

Attributes change now and then\. Events keep piling up\.

### 5. Events get their own table

Because events keep coming, each one is written as a new row in an events table\. Every event row carries Ana's tag, \#7, instead of a copy of everything about her\. Three events point to one entity\.

Designers call this many\-to\-one: many events, one customer\.

### 6. A join matches the tags

To answer a question, the warehouse joins tables\. It follows the tags on a sale to the matching rows in the Customers and Products tables, then sets the pieces side by side\. Join the first sale to see it\.

A join only matches tags while answering\. Nothing is moved or copied for good\.

### 7. A star of tables

With events in the middle and entities all around, the tables form a star\. Following the tags lets the warehouse answer big questions, like how many apples were bought in Lyon\.

Designers call this layout a star schema\.