SQL (Structured Query Language) is a programming language used to insert, update, delete, and query data stored in the tables of a relational database. It is a declarative language, which means that users describe what they want to do (instead of how they want it done), and the SQL engine figures out how to execute. It became the standard language for relational databases in the early 1970s. Today SQL is supported by all major relational databases including MySQL, PostgreSQL, Microsoft SQL Server, Oracle, and IBM DB2.
Scaling SQL databases is challenging. They were designed in a period when data (and the database) could live on a single machine. Therefore, if a SQL database needs to scale, more hardware must be purchased to add more processing power, memory, and storage. This becomes expensive.
To address the issues of scalability, NoSQL and NewSQL emerged in the early 2000s. However, compromises had to be made in other areas (consistency, ACID, etc.). This, in turn, led to the emergence of distributed SQL.