> For the complete documentation index, see [llms.txt](https://vikram-bajaj.gitbook.io/cs-gy-6083-principles-of-database-systems/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://vikram-bajaj.gitbook.io/cs-gy-6083-principles-of-database-systems/main-1/sql/advanced-sql/accessing-metadata.md).

# Accessing Metadata

The dictionary or catalog stores information about the database itself. This is data about data or 'metadata'.

It holds:

* Description of database objects (tables, users, rules, views, indexes,..)
* Information about who is using what data (locks)
* Schemas and mapping
* The dictionary itself

For example, consider the following relations:

ACTOR(**AID**, FIRSTNAME, LASTNAME)

MOVIE(**MID**, MNAME, BUDGET, GROSS)

ACTED\_IN(**AID**, **MID**, STARRING, WAGE)

**Listing all the tables that exist in the 'test' schema:**

```
SELECT *
FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_SCHEMA = 'TEST'
```

**Listing all tables that have more than 3 columns:**

```
SELECT TABLE_NAME
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_SCHEMA = 'TEST'
GROUP BY TABLE_NAME
HAVING COUNT(*) > 3
```

**Listing the first name and last name of all actors whose last name is equal to the name of an attribute in MOVIE:**

```
SELECT FIRSTNAME, LASTNAME
FROM TEST.ACTOR, INFORMATION_SCHEMA.COLUMNS I
WHERE TABLE_SCHEMA = 'TEST'
AND I.TABLE_NAME = 'MOVIE'
AND LASTNAME = I.COLUMN_NAME
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://vikram-bajaj.gitbook.io/cs-gy-6083-principles-of-database-systems/main-1/sql/advanced-sql/accessing-metadata.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.