Andrew Tobin explains the order of how a SQL statement gets processed:
For a particular query you may have the following components and they act in this order:
1 FROM / JOIN 2 WHERE 3 GROUP BY 4 HAVING 5 SELECT 6 ORDER BY 7 TOP / OFFSET-FETCH
Read on for more details. This is why, for example, you can use an aliased column or calculation in the ORDER BY clause but not in the WHERE clause.