What is the difference between structured, semi-structured, and unstructured data?

Structured data is highly organized and fits into relational tables with a defined schema. Semi‑structured data contains tags or markers that separate elements but does not enforce a rigid schema, such as JSON, XML, or CSV. Unstructured data has no predefined format, like images, audio, video, or free‑text logs. Structured data is ideal for OLTP and relational queries; semi‑structured data is common in data lakes and NoSQL stores; unstructured data is used in analytics, machine learning, and content management. The choice depends on the use case: if you need fast, transactional processing, use structured; if you need flexible schema and can tolerate schema evolution, use semi‑structured; if you need to store raw files or media, use unstructured.