SQL

Finding Records Without Related Entries using LEFT JOIN

Find records in one table that lack corresponding entries in another. Use LEFT JOIN with WHERE IS NULL for data cleanup, finding inactive users, or reports.

SELECT
    u.user_id,
    u.username,
    u.email
FROM users u
LEFT JOIN orders o ON u.user_id = o.user_id
WHERE o.order_id IS NULL;

How it works: This query identifies users who have not placed any orders. It uses a `LEFT JOIN` to combine every row from the `users` table with matching rows from the `orders` table. If a user has no corresponding order, the columns from the `orders` table for that user will be `NULL`. The `WHERE o.order_id IS NULL` condition then filters these results, returning only users who do not have any associated entries in the `orders` table.

Finding Records Without Related Entries using LEFT JOIN

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