eolas/Databases/SQL/1_Language_structure.md

categories	tags
Databases Programming Languages	SQL

SQL: Language structure

The structure or syntax of SQL is composite and mirrors that of a natural or logical language. There are overall statements (queries) which subdivide hierarchially into clauses, predicates, and expressions.

Query

• A statement that updates or retrieves data from a table based on passed-in criteria
• The highest level in the syntax tree, equivalent to a sentence in natural language
• Examples:
  • SELECT
  • UPDATE
  • DELETE
• Applied example:
  • SELECT name FROM model

Clause

• Typically provides parameters for a given query
• For example running a query against a specific table (FROM) that satisfies certain conditions (WHERE)

Predicates

• A logical or numerical condition which is passed to a given clause.
• For example WHERE a given condition obtains

Expression

• The lowest level of the hierarchy: field, row names etc