The most effective way to find all customers who have not placed an order is to use a LEFT JOIN. This query starts with the Customers table (the "left" table) and joins it to the Products table. A LEFT JOIN returns all records from the Customers table, regardless of whether there is a matching record in the Products table. If a customer has no orders, the columns from the Products table for that customer's row will be populated with NULL. The WHERE P.CustomerID IS NULL clause then filters the results to show only those rows where no match was found, effectively isolating the customers without orders.

B. This query uses an INNER JOIN, which would only return customers who have placed an order. It also contains multiple syntax errors, such as an invalid join condition and the use of an aggregate function (COUNT) in a WHERE clause.

C. This query also incorrectly uses an INNER JOIN, which would filter out customers without orders from the start. The WHERE clause contains a syntactically invalid subquery that attempts to use a boolean expression.

D. This query performs a LEFT JOIN from Products to Customers and filters for non-null CustomerIDs. This would return all orders and their associated customer information, which is the opposite of the desired outcome.

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1. Silberschatz

A.

Korth

H. F.

& Sudarshan

S. (2020). Database System Concepts (7th ed.). McGraw-Hill. In Chapter 4

"Intermediate SQL

" the section on "Outer-Join Expressions" describes how a LEFT OUTER JOIN preserves tuples from the left relation that do not have matching tuples in the right relation

extending them with null values. This is the foundational principle for the correct answer's logic.

2. PostgreSQL 16 Documentation. (2023). "7.2.2. JOIN Clauses". The documentation for LEFT OUTER JOIN states: "For each row R1 of T1

the joined table has a row for each row in T2 that satisfies the join condition with R1. ... If no join partner exists in T2

a row is generated with null values in the T2 columns." This directly supports the mechanism of using WHERE ... IS NULL to find non-matching rows.

3. SQLite Documentation. "The LEFT and CROSS JOIN keywords". The official documentation explains: "If the ON or USING clause of a LEFT JOIN is not satisfied for some row in the left-hand table

then a single row is output for that left-hand table row with NULL values in the columns that would have been filled from the right-hand table." This confirms the behavior exploited by the correct query.