dudas con alias

[deadlykyo]

Como tu dices el uso de la clausula AS es opcional pero como buena practica se recomienda el uso de la misma, mira lo que dice la documentacion de mysql:

A select_expr can be given an alias using AS alias_name. The alias is used as the expression's column name and can be used in GROUP BY, ORDER BY, or HAVING clauses. For example:

SELECT CONCAT(last_name,', ',first_name) AS full_name
FROM mytable ORDER BY full_name;

The AS keyword is optional when aliasing a select_expr. The preceding example could have been written like this:

SELECT CONCAT(last_name,', ',first_name) full_name
FROM mytable ORDER BY full_name;

However, because the AS is optional, a subtle problem can occur if you forget the comma between two select_expr expressions: MySQL interprets the second as an alias name. For example, in the following statement, columnb is treated as an alias name:

SELECT columna columnb FROM mytable;

For this reason, it is good practice to be in the habit of using AS explicitly when specifying column aliases.

It is not allowable to refer to a column alias in a WHERE clause, because the column value might not yet be determined when the WHERE clause is executed. See Section B.1.5.4, “Problems with Column Aliases”.

Atentamente,
Edson Gonzales.