Subqueries

Use EXISTS and NOT EXISTS for Relationship Checks in SQL

Many SQL queries do not actually need data from a related table. They only need to answer a yes-or-no question: Does this customer have at least one paid order? Is this project missing an active owner? Does any inventory row satisfy this product requirement? A common first attempt is to join the tables and then remove duplicates. That can work, but it makes the query produce more rows than the problem requires and then asks a later operation to repair the result.