Databases are used by many of today’s applications, especially Web-based applications. Most database systems are based on the concepts of relational database theory, so you can expect any problems you encounter to be about accessing and manipulating relational data. To do this you’ll need to understand basic SQL commands such as SELECT and INSERT.
Transactions are used to group multiple changes to a database together into a single unit such that either all the changes succeed and are made permanent (the transaction is committed) or all fail and are undone (the transaction is rolled back). The ACID acronym describes the four fundamental properties of a transaction: atomicity, consistency, isolation, and durability. Most relational databases support ACID transactions, although the isolation levels are often variable in order to allow for better performance.