What is a Data Schema?

In a relational database, a schema is the set of tables, columns, data types, and foreign-key relationships that constrain what can be stored and how records relate to each other. In document or API-based systems, a schema serves the same purpose in a looser form - defining the shape a JSON object is expected to take, often enforced by a validation library rather than the storage engine itself. Schemas act as a contract between producers and consumers of data: an integration or downstream pipeline is written against a specific schema, and when that schema changes without warning - a field renamed, a type changed from string to number - anything depending on the old shape can break silently rather than loudly. This is why schema versioning and migration discipline matter as much as the schema design itself. Schema design also has a direct compliance dimension: which fields exist, and whether they hold personal data, determines what protections (encryption, retention rules, erasure coverage) that model needs from day one.