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# 03. Three-Level Namespace in Unity Catalog

In this lesson, you will learn how to work with the **three-level namespace** in Databricks Unity Catalog. This structure allows you to organize your data assets in a hierarchical manner, using **catalog**, **schema**, and **table** levels.

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## 1. Use the Catalog

The **catalog** is the top-level namespace in Unity Catalog. It acts as a container for one or more schemas. To work within a specific catalog, you use the `USE CATALOG` command.

### ✅ Primary Operation
“`sql
USE CATALOG training;
“`

This command sets the default catalog for subsequent operations. Any schema or table referenced without a catalog name will be assumed to belong to the `training` catalog.

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## 2. Use a Schema

Within a catalog, you can have multiple **schemas**. A schema is like a folder that organizes tables, views, and other objects. To work within a specific schema, you use the `USE SCHEMA` command.

### ✅ Allowed SQL Step
“`sql
USE SCHEMA sales;
“`

This command sets the default schema for subsequent operations. Any table referenced without a schema name will be assumed to belong to the `sales` schema within the `training` catalog.

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## 3. Access Tables in the Three-Level Namespace

Once you have selected a catalog and a schema, you can access tables by using the fully qualified name, which consists of **catalog** > **schema** > **table**.

### ✅ Allowed SQL Step
“`sql
SELECT * FROM sales.customers;
“`

This query retrieves all data from the `customers` table located in the `sales` schema of the `training` catalog.

Alternatively, you can reference the same table using the fully qualified name:

### ✅ Allowed SQL Step
“`sql
SELECT * FROM training.sales.customers;
“`

This version explicitly specifies the catalog, schema, and table, making it useful when you want to avoid ambiguity or when working across multiple catalogs.

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## 4. Practical Example: Exploring Data

Let’s walk through a practical example to reinforce your understanding of the three-level namespace.

### Step 1: Use the catalog
“`sql
USE CATALOG training;
“`

### Step 2: Use the schema
“`sql
USE SCHEMA sales;
“`

### Step 3: Query the table
“`sql
SELECT * FROM customers;
“`

This query assumes you are currently in the `training` catalog and the `sales` schema, so it accesses the `customers` table directly.

Alternatively, you could use the fully qualified name:
“`sql
SELECT * FROM training.sales.customers;
“`

Both versions are valid and will return the same result.

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## 5. Summary

– The **three-level namespace** in Unity Catalog is made of **catalog**, **schema**, and **table**.
– Use `USE CATALOG training;` to set the catalog context.
– Use `USE SCHEMA sales;` to set the schema context.
– Access tables using either the short form (when in the correct context) or the fully qualified name.

By understanding and using the three-level namespace, you can better organize and access your data assets in Unity Catalog. This structure is essential for managing large-scale data environments in Databricks.