In HSQLDB, transactions are fundamental units of work that allow you to group one or more SQL statements into a single logical operation. The concept of transactions is based on the ACID (Atomicity, Consistency, Isolation, Durability) properties, which ensure the reliability and integrity of database operations. Here's an explanation of each aspect of transactions in HSQLDB:

Atomicity : Transactions in HSQLDB are atomic, meaning that they are either fully completed or fully aborted. If any part of a transaction fails (e.g., due to an error or exception), the entire transaction is rolled back, and any changes made by the transaction are undone, leaving the database in its original state. This ensures that the database remains consistent even in the presence of errors.

Consistency : Transactions in HSQLDB maintain the consistency of the database by ensuring that it transitions from one consistent state to another consistent state. This means that transactions preserve data integrity and enforce constraints, such as foreign key relationships and uniqueness constraints, during their execution.
Isolation : Transactions in HSQLDB operate in isolation from each other, meaning that the changes made by one transaction are not visible to other transactions until the first transaction is committed. This isolation prevents interference and ensures that transactions can execute concurrently without affecting each other's outcomes. HSQLDB provides different levels of isolation, such as Read Uncommitted, Read Committed, Repeatable Read, and Serializable, which allow you to control the trade-off between concurrency and consistency.

Durability : Transactions in HSQLDB are durable, meaning that once a transaction is committed, its changes are permanent and persist even in the event of a system failure or crash. HSQLDB achieves durability by ensuring that committed transactions are safely written to disk and logged in a transaction log before acknowledging their completion to the user. This ensures that the database can recover to a consistent state after a crash or restart.