SELECT name AS customers
FROM customers
WHERE (
        SELECT COUNT(*)
        FROM orders
        WHERE customerId = customers.id -- the "correlation" is here
        ) < 1

The given answer (which isn't necessarily the best possible solution) is using a "correlated subquery". This "correlation" takes place through the where clause of the subquery and it enables the subquery to "align" the subquery result (i.e. the count(*) value) to each row of the customers table, then that value is compared to <1.

Characteristics of a "correlated subquery"

• Nested queries: The SQL statement has a query within a query. The "inner query" (everything inside the parentheses) is nested within the "outer query".
• Reference to the "outer query": The "inner query" is referencing customers.id which comes from the "outer query". This is the key characteristic of a correlated subquery.
• Row-by-row execution: Correlated subqueries run once for each row processed by the outer query. In this case, for each customer, the database is counting the number of orders that customer has
• Performance: Correlated subqueries can be slower than other types of queries because they require multiple executions of the inner query

There are at least 2 other approaches for such a problem:

• using NOT EXISTS (also known as a "left semi-join"), or
• using a LEFT JOIN and WHERE clause (sometimes known as "left excluding join")

    -- using NOT EXISTS
    SELECT name AS customers
    FROM customers
    WHERE NOT EXISTS (
            SELECT 1
            FROM orders
            WHERE customerId = customers.id -- "semi-join" formed by this
            ) 
    ;
    
    -- using LEFT JOIN
    SELECT name AS customers
    FROM customers
    LEFT JOIN ORDERS ON orders.customerId = customers.id
    WHERE orders.customerId IS NULL