While the selection of a database and a hardware platform is a must, the selection of an ETL tool is highly recommended, but it's not a must. When you evaluate ETL tools, it pays to look for the following characteristics:
Functional capability: This includes both the 'transformation' piece and the 'cleansing' piece. In general, the typical ETL tools are either geared towards having strong transformation capabilities or having strong cleansing capabilities, but they are seldom very strong in both. As a result, if you know your data is going to be dirty coming in, make sure your ETL tool has strong cleansing capabilities. If you know there are going to be a lot of different data transformations, it then makes sense to pick a tool that is strong in transformation.
Ability to read directly from your data source: For each organization, there is a different set of data sources. Make sure the ETL tool you select can connect directly to your source data.
Metadata support: The ETL tool plays a key role in your metadata because it maps the source data to the destination, which is an important piece of the metadata. In fact, some organizations have come to rely on the documentation of their ETL tool as their metadata source. As a result, it is very important to select an ETL tool that works with your overall metadata strategy.   _{Your Comment}