---
title: "Introduction"
description: "What CAT is about and the basic terms"
url: "https://docs.justcat.it/reference/basics/introduction/"
---
# Introduction


## What is CAT about

**Data is prone to problems.** It has issues. Even if it doesn't, it is easy to screw it up. Data is often "travelling" through the company systems - e.g., records are created in a CRM system, loaded into DWH and from DWH to Power BI workspace for reporting. Data teams release new versions of these solutions. There is a big potential for things to go wrong.

It is a best practice to have **automated tests** that give you enough confidence that all is working and all the data is trustworthy. With CAT you achieve this easily and quickly and will have your data under control.

If you end up here, we expect you have some *data* and you want to *trust* it.

> **😺:** **CAT is about turning your *assumptions* about data into *confidence* -** you will not assume the data is correct, you will know it is. Or you will exactly know *what* is wrong. You achieve that very simply by creating very **simple automated tests**.



This is technical reference documentation — for the value behind CAT and its use cases, read [introduction to CAT](https://docs.justcat.it/what-is-cat/introduction/ "What is CAT").

## Basic terms

In CAT you need to define:

[Data sources](https://docs.justcat.it/reference/data-sources/ "Data sources")
: Where is the data you want to test?

[Queries](https://docs.justcat.it/reference/queries/ "Queries")
: Questions asked against your data sources — written directly in a test, or defined on their own under a name

[Tests](https://docs.justcat.it/reference/tests/ "Tests")
: Simple queries against data and your expectations about the results

[Outputs](https://docs.justcat.it/reference/outputs/ "Outputs")
: Optionally you can define the format for results of your tests (MS Excel, export to database and many others)

These definitions live in a [project file](https://docs.justcat.it/reference/project-file/ "Project file") — directly, or in other places the project file points to. Each linked section documents one definition's properties, wherever the definition is stored.

## Data sources

Every company has lots of data. Very often, the data is in various formats:

![Various data formats](data-sources.png)


A **Data Source** in CAT is a definition of where the data is and how to connect to it. You give the data source a friendly name (such as CRM, DWH, Power BI, ...). This friendly name is then used in your tests, where you define your expectation about the data in the system. This way you don't have to repeat all connection information in every test - you just specify that your test examines data in e.g., DWH.

For details, see [Data sources](https://docs.justcat.it/reference/data-sources/ "Data sources").

## Tests

A **TestDefinition** describes your *expectations* about the data. E.g., you expect that a SQL statement issued against your CRM system (MySQL) returns exactly the same data as another SQL statement issued against your DWH (SQL Server). Or that none of your tables in a Power BI model is empty.

Example of a simple smoke test:


**Properties**



Test Suite
: Smoke tests

Test Name
: Customer dimension table has active customers

Data Source
: DWH

Query
: ```sql
  SELECT *
  FROM Dimension.Customer
  WHERE IsActive = 1
  ```

Expectation
: set is not empty




**YAML**


```yaml
Tests:
- Test Suite:   Smoke tests
  Test Name:    Customer dimension table has active customers
  Data Source:  DWH
  Query: |
                SELECT *
                FROM Dimension.Customer
                WHERE IsActive = 1
  Expectation:  set is not empty
```





You can define your tests wherever you want - in YAML (as in the example), in MS Excel, in a relational database table. You can also very easily author them in the GUI - CAT Studio.

For details, see [Tests](https://docs.justcat.it/reference/tests/ "Tests").


## Outputs

Outputs are results of your tests in a format you need. You may or may not need outputs.

By default, every CAT tool shows test results in its own way:


CAT Studio
: displays test results directly in the GUI, in the grid where you have your tests and in the test detail

CAT cmdline tools
: stream test results to standard output and to log files



For that, you don't need to set anything - that always works. But there may be various situations, where you need test results also in other formats:

* Developers or business users often need results of tests in **MS Excel** format
* You want to easily build reports about how the data is doing, for your stakeholders - so you need test results in a **relational database table**
* **Azure DevOps or other automation tools** need special file formats in order to display nice charts and details of test results
* You may be tasked to automate further processes and you need test results in **JSON**
* and others.

And you may even need more outputs, not only one.

In CAT you can define these outputs in seconds - it really takes a few clicks in CAT Studio or a few lines in YAML.

For details, see [Outputs](https://docs.justcat.it/reference/outputs/ "Outputs").