1767. Find the Subtasks That Did Not Execute

Description

Table: Tasks

+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
| task_id        | int     |
| subtasks_count | int     |
+----------------+---------+
task_id is the column with unique values for this table.
Each row in this table indicates that task_id was divided into subtasks_count subtasks labeled from 1 to subtasks_count.
It is guaranteed that 2 <= subtasks_count <= 20.

 

Table: Executed

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| task_id       | int     |
| subtask_id    | int     |
+---------------+---------+
(task_id, subtask_id) is the combination of columns with unique values for this table.
Each row in this table indicates that for the task task_id, the subtask with ID subtask_id was executed successfully.
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: 
Tasks table:
+---------+----------------+
| task_id | subtasks_count |
+---------+----------------+
| 1       | 3              |
| 2       | 2              |
| 3       | 4              |
+---------+----------------+
Executed table:
+---------+------------+
| task_id | subtask_id |
+---------+------------+
| 1       | 2          |
| 3       | 1          |
| 3       | 2          |
| 3       | 3          |
| 3       | 4          |
+---------+------------+
Output: 
+---------+------------+
| task_id | subtask_id |
+---------+------------+
| 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.

SQL Code
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# Write your MySQL query statement below
WITH RECURSIVE
    T(task_id, subtask_id) AS (
        SELECT
            task_id,
            subtasks_count
        FROM Tasks
        UNION ALL
        SELECT
            task_id,
            subtask_id - 1
        FROM t
        WHERE subtask_id > 1
    )
SELECT
    T.*
FROM
    T
    LEFT JOIN Executed USING (task_id, subtask_id)
WHERE Executed.subtask_id IS NULL;