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Problems

Finding Customers with Orders: Leveraging the EXISTS Clause

Many real-world applications require identifying records in one table that have at least one corresponding record in another table. For example, you might want to find all customers who have placed at least one order, or all products that have been purchased. The EXISTS clause in SQL is a powerful and efficient tool for such scenarios, allowing you to check for the presence of related rows without necessarily retrieving them. This challenge will test your understanding and application of the EXISTS clause.

Problem Description

Your task is to write a SQL query that identifies all customers who have placed at least one order. You are provided with two tables: Customers and Orders. You should return a list of customer details for only those customers who appear in the Orders table. The key requirement is to use the EXISTS clause in your query to achieve this.

What needs to be achieved: Retrieve all information about customers who have placed at least one order.

Key requirements:

  • Utilize the EXISTS clause.
  • The query should be efficient.
  • Return all columns from the Customers table for matching customers.

Expected behavior: The query should return rows from the Customers table only for those customers whose customer_id exists in the Orders table. If a customer has multiple orders, they should still appear only once in the result set.

Important edge cases to consider:

  • Customers who have not placed any orders should not be included in the output.
  • The Orders table might be empty, in which case no customers should be returned.
  • The Customers table might be empty, in which case no customers should be returned.

Examples

Example 1:

Input:

Customers Table:

customer_idfirst_namelast_name
101AliceSmith
102BobJohnson
103CharlieBrown

Orders Table:

order_idcustomer_idorder_datetotal_amount
11012023-01-1550.00
21032023-02-2075.50
31012023-03-1025.00

Output:

customer_idfirst_namelast_name
101AliceSmith
103CharlieBrown

Explanation: Alice (customer_id 101) has two orders, and Charlie (customer_id 103) has one order. Bob (customer_id 102) has no orders. Therefore, only Alice and Charlie are included in the output.

Example 2:

Input:

Customers Table:

customer_idfirst_namelast_name
201DavidLee
202EveAdams

Orders Table:

order_idcustomer_idorder_datetotal_amount

Output: (Empty result set)

Explanation: The Orders table is empty. Therefore, no customers have placed any orders, and the output is an empty table.

Example 3:

Input:

Customers Table:

customer_idfirst_namelast_name
301FrankWhite

Orders Table:

order_idcustomer_idorder_datetotal_amount
103012023-04-01100.00
113012023-04-05120.00

Output:

customer_idfirst_namelast_name
301FrankWhite

Explanation: Frank (customer_id 301) has placed multiple orders, but since we are only checking for the existence of at least one order, he appears only once in the result.

Constraints

  • The Customers table will have at least 0 and at most 10,000 rows.
  • The Orders table will have at least 0 and at most 100,000 rows.
  • Both tables will contain columns named customer_id.
  • The customer_id column is of an integer type in both tables.
  • The query should execute within 1 second on the given constraints.

Notes

  • The EXISTS clause is generally more performant than using IN with a subquery when the subquery might return a large number of rows, as EXISTS stops scanning the subquery as soon as the first match is found.
  • Consider how the EXISTS clause is structured within the WHERE clause.
  • Think about the correlation between the outer query and the subquery.
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