SQL Exercises: Customer with more than one current order

Last update on December 20 2024 11:46:09 (UTC/GMT +8 hours)

9. From the following table, write a SQL query to find those salespersons and customers who have placed more than one order. Return ID, name.

Sample table: customer

Sample table: salesman

Sample table: orders

Sample Solution:

-- Selecting specific columns (customer_id as "ID", cust_name as "NAME") from the 'customer' table (aliased as 'a')
SELECT customer_id as "ID", cust_name as "NAME"

-- Filtering rows in the 'customer' table (aliased as 'a') where the count of associated orders is greater than 1
FROM customer a
WHERE 1 <
    (SELECT COUNT(*)
     FROM orders b
     WHERE a.customer_id = b.customer_id)

-- Performing a UNION operation with the result set of a subquery
UNION

-- Selecting specific columns (salesman_id as "ID", name as "NAME") from the 'salesman' table (aliased as 'a')
SELECT salesman_id as "ID", name as "NAME"

-- Filtering rows in the 'salesman' table (aliased as 'a') where the count of associated orders is greater than 1
FROM salesman a
WHERE 1 <
    (SELECT COUNT(*)
     FROM orders b
     WHERE a.salesman_id = b.salesman_id)

-- Ordering the result set based on the second column ("NAME")
ORDER BY 2

Sample Output:

ID	        NAME
3009		Geoff Cameron
3005		Graham Zusi
5001		James Hoog
5003		Lauson Hen
5002		Nail Knite
3002		Nick Rimando

Code Explanation:

The said query in SQL which selects customer_id and cust_name columns from the customer table and renames them as "ID" and "NAME" respectively.
The subquery checks whether at least one order placed by each customer or not. If the count of orders for a customer is greater than 1, then that customer is included in the results.
The UNION operator is used to combine the results of this query with another query that selects salesman_id and name columns from the salesman table, renames them as "ID" and "NAME" respectively, and checks if there is at least one order placed by each salesman. The final result is then ordered by the "NAME" column.

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