# 1767. Find the Subtasks That Did Not Execute

## Description

Table: Tasks

+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
+----------------+---------+
task_id is the column with unique values for this table.
It is guaranteed that 2 <= subtasks_count <= 20.


Table: Executed

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
+---------------+---------+
(task_id, subtask_id) is the combination of columns with unique values for this table.
It is guaranteed that subtask_id <= subtasks_count for each task_id.

Write a solution to report the IDs of the missing subtasks for each task_id.

Return the result table in any order.

The result format is in the following example.

Example 1:

Input:
+---------+----------------+
+---------+----------------+
| 1       | 3              |
| 2       | 2              |
| 3       | 4              |
+---------+----------------+
Executed table:
+---------+------------+
+---------+------------+
| 1       | 2          |
| 3       | 1          |
| 3       | 2          |
| 3       | 3          |
| 3       | 4          |
+---------+------------+
Output:
+---------+------------+
+---------+------------+
| 1       | 1          |
| 1       | 3          |
| 2       | 1          |
| 2       | 2          |
+---------+------------+
Explanation:
Task 1 was divided into 3 subtasks (1, 2, 3). Only subtask 2 was executed successfully, so we include (1, 1) and (1, 3) in the answer.
Task 2 was divided into 2 subtasks (1, 2). No subtask was executed successfully, so we include (2, 1) and (2, 2) in the answer.
Task 3 was divided into 4 subtasks (1, 2, 3, 4). All of the subtasks were executed successfully.


## Solutions

Solution 1: Recursive Table Generation + Left Join

We can generate a table recursively that contains all pairs of (parent task, child task), and then use a left join to find the pairs that have not been executed.

• # Write your MySQL query statement below
WITH RECURSIVE
SELECT
UNION ALL
SELECT
FROM t