FAQ: Set Operations - Union All


#1

This community-built FAQ covers the “Union All” exercise from the lesson “Set Operations”.

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This exercise can be found in the following Codecademy content:

SQL: Table Transformation

FAQs on the exercise Union All

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#2

I understand that the query is counting the number of order items in the union of the two tables, but I’m struggling to get what the “as a” means at the end:

SELECT count(*) FROM (
SELECT id, sale_price FROM order_items
UNION ALL
SELECT id, sale_price FROM order_items_historic) as a;

I saw something similar in earlier lessons, but couldn’t find an explanation there either.

Any help would be appreciated!

Thanks


#3

The ‘as a’ is an SQL alias or ‘nickname’ of sorts that’s used to give a table, a column, or a subquery a temporary name that will be used in the query. The name allows for one to easily understand and define which table or subquery is being referenced by each column. It can also be used to give a column a name different from the one provided in the table so that the results reflect that alias, which is sometimes useful when the orginal column name in a table isn’t clear or readable.