An ETL diagram is a visual representation of the extract, transform, and load process that is used to collect data from various sources, transform it according to business rules, and load it into a destination data store¹. An ETL diagram can help to illustrate the data flow, the data sources and destinations, the transformation logic, and the dependencies among the tasks².

There are different ways to create an ETL diagram, but one common approach is to use a data flow diagram (DFD) notation. A DFD consists of four main symbols²:

• External entities: These are the sources or destinations of data outside the system, such as databases, files, or applications. They are represented by rectangles with rounded corners.

• Processes: These are the tasks or operations that transform the data, such as filtering, sorting, aggregating, or joining. They are represented by circles or ovals with a name inside.

• Data stores: These are the places where data is stored within the system, such as staging tables or temporary files. They are represented by parallel lines with a name above.

• Data flows: These are the paths or directions of data movement between the entities, processes, and data stores. They are represented by arrows with a name above.

Here is an example of an ETL diagram using DFD notation²:

The diagram shows how data is extracted from three different sources: a customer database, a product database, and a sales file. The data is then transformed by various processes, such as joining customer and product data, calculating sales metrics, and validating data quality. The transformed data is then loaded into two different data stores: a sales warehouse and a sales report.