The data in a Data Warehouse system is loaded with an ETL (Extract, Transform, Load) tool. As the name suggests, it performs the following three operations −
Extracts the data from your transactional system which can be an Oracle, Microsoft, or any other relational database,
Transforms the data by performing data cleansing operations, and then
Loads the data into the OLAP data Warehouse.
You can also extract data from flat files like spreadsheets and CSV files using an ETL tool and load it into an OLAP data warehouse for data analysis and reporting.