SQL Server - Logical Query Processing Phases

The main statement used to retrieve data in T-SQL is the SELECT statement. Following are
the main query clauses specified in the order that you are supposed to type them (known as
“keyed-in order”):
1. SELECT
2. FROM
3. WHERE
4. GROUP BY
5. HAVING
6. ORDER BY
But as mentioned, the logical query processing order, which is the conceptual interpretation
order, is different. It starts with the FROM clause. Here is the logical query processing
order of the six main query clauses:
1. FROM
2. WHERE
3. GROUP BY
4. HAVING
5. SELECT
6. ORDER BY
1.What is the difference between the WHERE and HAVING clauses?
The WHERE clause is evaluated before rows are grouped, and therefore is evaluated
per row. The HAVING clause is evaluated after rows are grouped, and therefore
is evaluated per group.

2. Why are you not allowed to refer to a column alias defined by the SELECT
clause in the WHERE clause?
Because the WHERE clause is logically evaluated in a phase earlier to the one
that evaluates the SELECT clause.

3. Why are you not allowed to refer to a column alias defined by the SELECT
clause in the same SELECT clause?
Because all expressions that appear in the same logical query processing phase
are evaluated conceptually at the same point in time.