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Discovering SQL: A Hands-On Guide for Beginners by Alex Kriegel

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WHAT IS SQL?

Before the advent of commercially available databases, every system in need of persistent storage had no choice but to implement its own, usually in some proprietary file format (binary or text) that only this application could read from and write to. This required every application that used these files to be intimately familiar with the structure of the file, which made switching to a different storage all but impossible. Additionally, you had to learn a vendor-specific access mechanism to be able to use it. Relational model dealt with complexities of data structures, organizing data on logical level, but it had nothing to say about the specifics of storage and retrieval except that it had to be set-based and follow relational algebra rules. Left to their own devices, the early RDBMSs implemented a number of languages, including SEQUEL, developed by Donald D. Chamberlin and Raymond F. Boyce in the early 1970s while working at IBM; and QUEL, the original language of Ingres. Eventually these efforts converged into a workable SQL, the Structured Query Language.

SQL is a RDBMS programming language designed to define relational constructs (such as schemas and tables) and provide data manipulation capabilities. Unlike many programming languages in general use, it does not exist outside the relational model. It cannot create stand-alone programs; it can only be used inside RDBMSs. This is a declarative type of language. It instructs the database about what you want to do, ...

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