Customers Who Never Order
LeetCode 183 | Difficulty: Easy
EasyProblem Description
Table: Customers
+-------------+---------+
| Column Name | Type |
+-------------+---------+
| id | int |
| name | varchar |
+-------------+---------+
id is the primary key (column with unique values) for this table.
Each row of this table indicates the ID and name of a customer.
Table: Orders
+-------------+------+
| Column Name | Type |
+-------------+------+
| id | int |
| customerId | int |
+-------------+------+
id is the primary key (column with unique values) for this table.
customerId is a foreign key (reference columns) of the ID from the Customers table.
Each row of this table indicates the ID of an order and the ID of the customer who ordered it.
Write a solution to find all customers who never order anything.
Return the result table in any order.
The result format is in the following example.
Example 1:
Input:
Customers table:
+----+-------+
| id | name |
+----+-------+
| 1 | Joe |
| 2 | Henry |
| 3 | Sam |
| 4 | Max |
+----+-------+
Orders table:
+----+------------+
| id | customerId |
+----+------------+
| 1 | 3 |
| 2 | 1 |
+----+------------+
Output:
+-----------+
| Customers |
+-----------+
| Henry |
| Max |
+-----------+
Topics: Database
Approach
Direct Approach
This problem can typically be solved with straightforward iteration or simple data structure usage. Focus on correctness first, then optimize.
When to use
Basic problems that test fundamental programming skills.
Solutions
Solution 1: MS SQL (Best: 545 ms)
| Metric | Value |
|---|---|
| Runtime | 545 ms |
| Memory | N/A |
| Date | 2018-04-14 |
Solution
/* Write your T-SQL query statement below */
select Name AS Customers
FROM Customers C
WHERE C.Id IN
(SELECT Id FROM Customers
EXCEPT SELECT DISTINCT CustomerId FROM Orders)
Complexity Analysis
| Approach | Time | Space |
|---|---|---|
| Solution | $O(n)$ | $O(1) to O(n)$ |
Interview Tips
Key Points
- Start by clarifying edge cases: empty input, single element, all duplicates.