SQL is an acronym for Structured Query Language. SQL is the standard language for communicating with relational databases. Short requests, called queries, are made to an SQL-compliant database and results are returned. Usually, these results are answers to the query, and contain the data requested. Other times, the results may be confirmation or error messages depending on the nature of the query. Rarely are queries and results handled directly by end users, rather, high level programing languages such as C, Python, or PHP provide an interface. These interfaces create the queries, interpret the results, and present the information returned in a format appropriate for the end user.
SQL is designed not only to retrieve data from a database, but also to insert and update data. Thus, SQL implementations often have write privileges to the database, which opens potential security holes. SQL queries intended to write to the database must be carefully constructed to prevent data contamination. Additionally, as SQL must be parsed by a high-level language, incompatibility issues regarding control characters can lead to accidental or deliberate malicious SQL queries being sent to the database. Even in instances in which an SQL implementation has no write privileges, techniques such as SQL injection can be used to access and read data in which the user is not supposed to have read privileges. Thus, as with all language-within-a-language situations, careful checks of data input from the user must be performed and strict filters must be in place.
SQL was first conceived and developed in the early 1970s at IBM. Called SEQUEL (Structured English Query Language) in early versions, the acronym was shorted to SQL in reflection of it's increasing complexity and to prevent trademark infringement issues. Oracle introduced the first commercial implementation of SQL in 1979, followed by IBM's offering later that same year. The American National Standards Institute approved a very small, strict version of SQL as an official standard in 1986, and ISO certification was approved the following year. The most widespread SQL implementation, SAG-SQL, was published by ANSI in 1991. Common SQL-compliant databases in use today include MySQL, Microsoft's Access and Server SQL, Oracle, SQLite, and PostgreSQL.
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