What Is SQL (Structured Query Language)

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SQL is a standard language for accessing databases.
You can use SQL to access and manipulate data in: MySQL, SQL Server, Access, Oracle, Sybase, DB2, and other database systems.

 

SQL (Structured Query Language) is a special-purpose used in programming and designed for managing data held in a (RDBMS), or for stream processing in a relational data stream management system (RDSMS).

Design

SQL deviates in several ways from its theoretical foundation, the relational model and its tuple calculus. In that model, a table is a set of tuples, while in SQL, tables and query results are lists of rows: the same row may occur multiple times, and the order of rows can be employed in queries (e.g. in the LIMIT clause).

Critics argue that SQL should be replaced with a language that strictly returns to the original foundation: for example, see The Third Manifesto.

The SQL language is subdivided into several language elements, including:

  1. Clauses, which are constituent components of statements and queries.
  2. Expressions, which can produce either scalar values, or tables consisting of columns and rows of data
  3. Predicates, which specify conditions that can be evaluated to SQL three-valued logic (3VL) (true/false/unknown) or Boolean truth values and are used to limit the effects of statements and queries, or to change program flow.
  4. Queries, which retrieve the data based on specific criteria.
  5. Statements, which may have a persistent effect on schemata and data, or may control transactions, program flow, connections, sessions, or diagnostics.
  6. Statements also include the semicolon (“;”) statement terminator. Though not required on every platform, it is defined as a standard part of the SQL grammar.
  7. Insignificant whitespace is generally ignored inĀ  statements and queries, making it easier to formatĀ  code for readability.

 

Example:


SELECT * FROM Customers;

 

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