In the early days of information technology (1950s – 1970s), computers were mostly mainframes and the information mostly structured data managed by information systems based on hierarchical and then relational databases.
With the emergence of descriptive markup languages such as SGML, XML, and JSON that add structure other forms of unstructured data or content such as text and streaming data, as well as NoSQL and graph database, linked data, and knowledge graph technologies, the distinction between structured and unstructured data or content is less relevant. Modern data lakes store structured, semi-structured, and unstructured data.
