Projections and Restrictions

A projection is a subset of the columns in a table. A restriction (also called selection) is a subset of the rows in a table, based on some conditions.

For example, the following SELECT statement retrieves the names and prices of all products that cost more than fifteen dollars:

SELECT Name, UniPrice
  FROM Products
WHERE UnitPrice > 15 

This query uses both a restriction (WHERE unit_price > 15) and a projection (SELECT name, unit_price)