In Looker, a derived table is a query whose results are used as if it were an actual table in the database.
For example, let’s say we have a database table called orders that has many columns. We can create a derived table named customer_order_summary and include a subset of the orders table’s columns:
We can now work with the customer_order_summary derived table just as if it were any other table in our database.
The Derived tables in Looker documentation page details the followings: