I used SQL UNION ALL to append multiple tables in one table. How can I assign a name for the new table?

I am a SQL beginner working on the Google Data Analytics Capstone project. There are 12 tables I need to analyze, with 13 columns and >5 million combined number of rows. I am running my queries in Google BigQuery and used the UNION ALL statement to append the tables in one new table. Now, I want to create a new column from the difference between column3 and column4. How can I do this without repeating the formula in the select statement of each of the 12 tables? Is there a way I can assign a name or a variable(?) for the new table so my code would look cleaner?

I tried creating a temporary table but it did not work and now I am stuck.

You can use CTE for this:

WITH MY_TABLE as (
    -- you query with union alls
)
select *, column3-column4 new_col from MY_TABLE 

But this is not a table. All you r inner queries will be executed once you run it.

If you want real table, I believe you could create table something like this:

create table as (
    -- you query with union alls
)

and later query this new table as usual.