Solution Explanation

This problem requires filtering orders based on whether a customer has placed an order of type 0. If a customer has at least one type 0 order, we should exclude all their type 1 orders from the results. Otherwise, all their orders are included.

Two SQL solutions are provided, both achieving the same outcome but with different approaches.

Solution 1: Using a Common Table Expression (CTE)

This solution employs a CTE (T) to identify customers with at least one type 0 order.

Steps:

1. CTE T: This CTE selects distinct customer IDs from the Orders table where order_type is 0. This efficiently gets a list of all customers who have placed type 0 orders.

2. Main Query: The main query selects all orders from the Orders table. The WHERE clause filters the results:

   • order_type = 0: This includes all orders of type 0, regardless of the customer's other order types.
   • NOT EXISTS(...): This condition checks if the current order's customer ID is not present in the T CTE. If it's not in T, it means the customer has no type 0 orders, so all their orders (including type 1) are included.

MySQL Code:

WITH
    T AS (
        SELECT DISTINCT customer_id
        FROM Orders
        WHERE order_type = 0
    )
SELECT *
FROM Orders AS o
WHERE order_type = 0 OR NOT EXISTS (SELECT 1 FROM T AS t WHERE t.customer_id = o.customer_id);

Time Complexity: The CTE has a time complexity of O(N), where N is the number of rows in the Orders table (due to the SELECT DISTINCT). The main query also has a time complexity of O(N) in the worst case (dependent on the index used). The overall time complexity is O(N).

Solution 2: Using a LEFT JOIN

This solution uses a LEFT JOIN to compare orders of a customer with different order types.

Steps:

1. LEFT JOIN: A LEFT JOIN is performed between the Orders table (aliased as a) and itself (aliased as b). The join condition is based on matching customer_id and different order_type. This essentially pairs each order with any orders of a different type from the same customer.

2. WHERE Clause: The WHERE clause filters the results:

   • b.order_type IS NULL: This condition selects orders where there's no matching order of a different type. This means the customer only has orders of one type (either all type 0 or all type 1).
   • b.order_type = 1: This condition selects type 1 orders only if there is no corresponding type 0 order for that customer.

3. SELECT DISTINCT: This ensures that only unique order rows are returned.

MySQL Code:

SELECT DISTINCT
    a.order_id,
    a.customer_id,
    a.order_type
FROM
    Orders AS a
    LEFT JOIN Orders AS b ON a.customer_id = b.customer_id AND a.order_type != b.order_type
WHERE b.order_type IS NULL OR b.order_type = 1;

Time Complexity: The LEFT JOIN operation has a time complexity that depends on the database's query optimizer and the presence of indexes. In the worst case, it could be O(N^2), but with appropriate indexes (on customer_id and order_type), it will likely perform in O(N log N) or even O(N). The DISTINCT clause adds some overhead but doesn't significantly change the overall complexity.

Conclusion:

Both solutions effectively solve the problem. Solution 1 (using CTE) might be slightly easier to understand conceptually, while Solution 2 (using LEFT JOIN) might offer better performance with appropriate indexing in some database systems. The choice between them depends on factors such as readability preferences and the specific characteristics of the database system being used.