In the database history article "What Goes Around Comes Around," (see Resources) Michal Stonebraker describes in detail how storage techniques have evolved over time. Before arriving at the relational model, developers tried other models such as hierarchical and directed graph. It is worth noting that the SQL-based relational model—which is the de facto standard even now—has prevailed for about 30 years. Given the short history and fast pace of computer science, this is a remarkable achievement. The relational model is so well-established that for many years, selecting data storage for an application was an easy choice for the solution architect. The choice was invariably a relational database.
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