with

With statements or Common Table Expressions.

Postgres docs.

Examples

WITH major AS (
    SELECT
        id,
        name
    FROM person
    WHERE age > 18
)

SELECT 
  pet.name,
  major.name
FROM major
INNER JOIN pet ON pet.person_id = person.id

WITH regional_sales AS (
    SELECT
        region,
        SUM(amount) AS total_sales
    FROM orders
    GROUP BY region
),

top_regions AS (
    SELECT region
    FROM regional_sales
    WHERE total_sales > (SELECT SUM(total_sales)/10 FROM regional_sales)
)

SELECT 
    region,
    product,
    SUM(quantity) AS product_units,
    SUM(amount) AS product_sales
FROM orders
WHERE region IN (SELECT region FROM top_regions)
GROUP BY region, product;