How SQL ETL Works
The ETL pipeline consists of three distinct phases. While modern tools automate much of this process, the fundamental architecture remains the same:
Extract: Data is pulled from various source systems. These sources can be incredibly diverse, ranging from legacy on-premises databases (like SQL Server or Oracle) to modern SaaS applications (like Salesforce or Workday) and flat files.
Transform: This is the core of SQL ETL. The extracted data is temporarily held in a staging area where SQL commands are used to clean, filter, join, aggregate, and format the data. This ensures consistency (e.g., converting all date formats to YYYY-MM-DD) and applies specific business logic before the data ever reaches the warehouse.
Load: The fully transformed, analytics-ready data is written into the target destination, such as a cloud data warehouse (Snowflake, Google BigQuery, or Amazon Redshift).
