Databases and SQL

This unit covers relational databases and the SQL used to work with them, including retrieving, sorting, filtering, combining and editing data.

Understand the structure before the syntax. A relational database splits data across linked tables so nothing is stored twice, with a primary key identifying each record uniquely and a foreign key creating the link. Once that is clear, SQL is mostly a matter of following fixed conventions. The clause order is not optional — SELECT, FROM, WHERE, ORDER BY — and writing them in any other order is a syntax error. Text values need quotation marks while numbers do not. The one to be careful with is UPDATE and DELETE: omitting the WHERE clause applies the statement to every record in the table.