Topic

SQL

SQL is a language for defining, querying, and changing data in relational database systems.

At a glance

Read operation
SELECT
Common data structure
Tables of rows and columns

Overview

SQL is a language for defining, querying, and changing data in relational database systems. A query describes a result in terms of tables and operations, while the database chooses an execution approach. Individual systems add their own syntax and capabilities.

Describing a result

Queries can select columns, filter rows, join related tables, and aggregate values. The result depends on the query's conditions and grouping, so a syntactically valid statement can still answer the wrong business question.

Changes and constraints

SQL also defines structures and changes records. Constraints can enforce data rules, and transactions group operations. Applications should bind external values through parameterized interfaces instead of assembling them into query text.

Sources and review

MOOR's explanatory text is supported by the following source links.

  1. The SQL language tutorial — PostgreSQL
  2. SQL language reference — SQLite

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