How do databases ensure data consistency?

Databases ensure data consistency through mechanisms like ACID properties, constraints, and normalisation.

ACID (Atomicity, Consistency, Isolation, Durability) properties are a set of principles that guarantee that database transactions are processed reliably. Atomicity ensures that all operations within a transaction are completed successfully; if not, the transaction is aborted. Consistency ensures that a transaction brings the database from one valid state to another, maintaining the integrity constraints. Isolation ensures that concurrent execution of transactions leaves the database in the same state as if the transactions were executed sequentially. Durability guarantees that once a transaction has been committed, it will remain committed even in the case of a system failure.

Constraints are rules enforced on data columns in a table. They ensure the accuracy and reliability of the data in the table and can be column level or table level. Types of constraints include NOT NULL, CHECK, DEFAULT, UNIQUE, PRIMARY Key, and FOREIGN Key. For example, a PRIMARY Key constraint uniquely identifies each record in a table and must contain unique and not null values. A FOREIGN Key constraint prevents actions that would destroy links between tables.

Normalisation is a database design technique that reduces data redundancy and eliminates undesirable characteristics like Insertion, Update and Deletion Anomalies. It divides larger tables into smaller tables and links them using relationships. The main aim of normalisation is to add, delete or modify a field that can be made in a single table, then propagated through the rest of the database via the defined relationships.

In addition to these, databases also use techniques like locking, which prevents multiple users from updating the same data at the same time, and versioning, which keeps track of changes made to the data over time. These mechanisms, along with ACID properties, constraints, and normalisation, help databases maintain data consistency.