MEPX
Chapter 1 of 10All chapters
  1. 1What SQL is
  2. 2SELECT and WHERE
  3. 3NULL
  4. 4Sorting and limiting
  5. 5Aggregates and GROUP BY
  6. 6Joins
  7. 7Keys and relationships
  8. 8Subqueries and CTEs
  9. 9Indexes and performance
  10. 10Changing data

Chapter 1 of 10

What SQL is

Declarative queries against tables.

Describing the result

SQL says what you want back, not how to fetch it. The query planner decides the strategy, which is why the same query can be fast on one database and slow on another.

  • Data lives in tables of rows and columns, each column with a type.
  • The core dialect is shared; every database then adds its own extensions.

The main statements

SELECT reads, INSERT adds, UPDATE changes and DELETE removes. Reading is most of the work, so most of this course is about SELECT.