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Problem

Table: Customers

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| customer_id   | int     |
| customer_name | varchar |
+---------------+---------+
customer_id is the primary key for this table.
Each row of this table contains the name and the id customer.

Write an SQL query to find the missing customer IDs. The missing IDs are ones that are not in the Customers table but are in the range between 1 and the maximum customer_id present in the table.

Notice that the maximum customer_id will not exceed 100.

Return the result table ordered by ids in ascending order.

The query result format is in the following example.

Example 1:

Input: 
Customers table:
+-------------+---------------+
| customer_id | customer_name |
+-------------+---------------+
| 1           | Alice         |
| 4           | Bob           |
| 5           | Charlie       |
+-------------+---------------+
Output: 
+-----+
| ids |
+-----+
| 2   |
| 3   |
+-----+
Explanation: 
The maximum customer_id present in the table is 5, so in the range [1,5], IDs 2 and 3 are missing from the table.

Code

WITH RECURSIVE id_seq AS (
    SELECT 1 as continued_id
    UNION 
    SELECT continued_id + 1
    FROM id_seq
    WHERE continued_id < (SELECT MAX(customer_id) FROM Customers) 
)

SELECT continued_id AS ids
FROM id_seq
WHERE continued_id NOT IN (SELECT customer_id FROM Customers)