Why doesn't LENGTH accept a new column alias as an argument in SQL?

Question

SQL

There is a table in the database:

id       first_name 
-------  ----------
3001     Пётр
3002     Василий
3003     Олег
3004     Дмитрий
3005     Евгений

I am writing an sql query that would display the phrase "Name is <here is the username>" and the length of this phrase:

select 'Name is ' || first_name as new_name, length('Name is ' || first_name) from table;

Conclusion:

Name is Пётр      | 12
Name is Василий   | 15
Name is Олег      | 12
...

I want to write a shorter query by making an alias to the first column and passing it to the length function parameter:

select 'Name is ' || first_name as new_name, length(new_name) from table;

One throws an exception:

"NEW_NAME": недопустимый идентификатор
 "%s: invalid identifier"

Is it possible to shorten the query length in this case? Why doesn't lenght accept a new column alias as an argument?

Answer 1

Wrap in a subquery and everything will be OK

SELECT new_name, length(new_name) FROM (
  select 'Name is ' || first_name as new_name from table
);

Example (for MySQL)