Structured Query Language

SQL covers table definitions, constraints, updates, transactions, and queries. A statement can describe selected columns, required conditions, and combinations of information while the database arranges execution. This emphasizes the desired data result rather than the procedural details of opening files and visiting individual storage positions one after another.

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A SELECT statement reads from tables or other queries, filters rows, and combines sources through joins. Grouping and aggregation calculate results for collections of rows. Filtering individual records and filtering aggregated groups express different questions, so a useful query begins with a clear account of the result it should produce.

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Tables can define column types, keys, and additional constraints that updates must obey. Transactions group related statements. Applications should pass values through parameters rather than assembling arbitrary SQL fragments from input, preserving a clear distinction between the structure of a command and the data supplied to that command.

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SQL has common standards, but systems differ in functions, types, and supported syntax. SQLite documents both omissions and extensions. Porting therefore requires checking actual semantics. Likewise, a query without ORDER BY does not promise a stable output sequence merely because repeated test runs happen to display its rows in the same order.

References: [2]