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)
