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Applications & Data · Databases

SQL

The declarative language used to define, query and manipulate relational data.

FoundationalUpdated 2026-09-01

Overview

SQL separates what you want from how it is retrieved. The engine's optimiser chooses an execution plan, which is why the same logical query can perform very differently depending on indexes, statistics and data distribution.

For IT operations, a working knowledge of SELECT, JOIN, GROUP BY and execution plans is enough to diagnose most reported 'the application is slow' complaints.

Security considerations

  • Parameterise queries; string concatenation is the root of SQL injection.
  • Grant least privilege, application accounts rarely need schema modification rights.