List customers with at least one order
Return each customer who has ordered exactly once in the result, regardless of how many orders they placed. Return only the requested columns and follow the specified ordering. This question probes an inner join excludes unmatched customers, then distinct removes join multiplicity.
Input
Setup script defining customers(id INT, name VARCHAR(30)); orders(id INT, customer_id INT, amount INT, order_date VARCHAR(10)).
Output
id, name; ascending customer ID.
Constraints
0–10000 rows per table. IDs are unique and non-NULL unless stated otherwise; foreign keys may be NULL. Numeric amounts are integers from 0 to 1000000. Dates are valid YYYY-MM-DD text. Text uses case-sensitive comparison; test data has consistent case. Duplicate business values and NULLs are allowed unless excluded by the task.
Example
Tables and sample rows
CREATE TABLE customers(id INT, name VARCHAR(30)); INSERT INTO customers VALUES (1,'Ana'),(2,'Ben'),(3,'Cara'); CREATE TABLE orders(id INT, customer_id INT, amount INT, order_date VARCHAR(10)); INSERT INTO orders VALUES (1,1,100,'2024-01-01'),(2,1,200,'2024-02-01'),(3,2,100,'2025-01-01'),(4,NULL,50,'2024-12-31');
Expected result
id | name 1 | Ana 2 | Ben
Another dataset (1)
CREATE TABLE customers(id INT, name VARCHAR(30)); INSERT INTO customers VALUES (1,'Solo'),(2,'None'); CREATE TABLE orders(id INT, customer_id INT, amount INT, order_date VARCHAR(10)); INSERT INTO orders VALUES (1,1,0,'2023-12-31'),(2,NULL,10,'2024-01-01');
Expected result
id | name 1 | Solo
Covers missing matches, ties, or NULL values without relying on insertion order.
Hints (2)
- Use an INNER JOIN to require a match.
- Remove duplicate customer rows after joining.
Target complexity: time O(n) · space O(n)