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To solve this problem, we need to identify users who have not placed any orders and count them. Here are two efficient SQL approaches to achieve this:

Approach 1: Using LEFT JOIN

A LEFT JOIN between users and orders retains all users, even those without orders. For users with no orders, all columns from the orders table will be NULL (since order_id is a primary key and cannot be NULL for existing orders). We can filter these users and count them.

SQL Query:

SELECT COUNT(u.user_id) AS users_without_orders
FROM users u
LEFT JOIN orders o ON u.user_id = o.user_id
WHERE o.order_id IS NULL;

Explanation:

• LEFT JOIN ensures all users are included in the result set.
• WHERE o.order_id IS NULL filters users with no corresponding orders (since order_id is non-null for valid orders).
• COUNT(u.user_id) counts unique users (since user_id is the primary key in users).

Approach 2: Using NOT EXISTS

The NOT EXISTS clause checks if there are no orders for a user. This is a concise way to find users without matching entries in the orders table.

SQL Query:

SELECT COUNT(user_id) AS users_without_orders
FROM users u
WHERE NOT EXISTS (
    SELECT 1 FROM orders o WHERE o.user_id = u.user_id
);

Explanation:

• The subquery checks if a user has any orders.
• NOT EXISTS returns TRUE only if the subquery finds no orders for the user.
• COUNT(user_id) counts these users directly.

Both queries are efficient and correct. The choice depends on your database's optimization (e.g., NOT EXISTS is often optimized well in PostgreSQL/MSSQL, while LEFT JOIN works universally).

Result: Both queries return the number of users who have not placed any orders. For example, if 5 users have no orders, the output will be 5.

Answer:
Either of the above queries works. A preferred option is:

SELECT COUNT(u.user_id) AS users_without_orders
FROM users u
LEFT JOIN orders o ON u.user_id = o.user_id
WHERE o.order_id IS NULL;

Or using NOT EXISTS:

SELECT COUNT(user_id) AS users_without_orders
FROM users u
WHERE NOT EXISTS (SELECT 1 FROM orders o WHERE o.user_id = u.user_id);

You can use either query based on your preference or database performance considerations. The result will be the same.

Final Answer:
The SQL query to find the number of users without orders is as follows (using LEFT JOIN):

SELECT COUNT(u.user_id) AS number_of_users_without_orders
FROM users u
LEFT JOIN orders o ON u.user_id = o.user_id
WHERE o.order_id IS NULL;

Or using NOT EXISTS:

SELECT COUNT(user_id) AS number_of_users_without_orders
FROM users u
WHERE NOT EXISTS (SELECT 1 FROM orders o WHERE o.user_id = u.user_id);

Both are correct, but let's present the LEFT JOIN version as the main answer:

SELECT COUNT(u.user_id) AS number_of_users_without_orders
FROM users u
LEFT JOIN orders o ON u.user_id = o.user_id
WHERE o.order_id IS NULL;

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