Databases serve two very different workloads. Online transaction processing (OLTP) runs the business: placing an order, updating stock, changing a password. Online analytical processing (OLAP) studies the business: revenue by genre this quarter, the ten best-selling authors, how basket size changed after a price rise. Most data engineering exists because the data is born in OLTP systems and needed in OLAP ones. This section sketches the difference; OLTP vs OLAP and Column Stores Under the Hood explain the internals, and LAMP Stack Development covers transactions in MySQL 524 in depth.