Formatted question description: https://leetcode.ca/all/1767.html

1767. Find the Subtasks That Did Not Execute

Level

Hard

Description

Table: Tasks

+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
| task_id        | int     |
| subtasks_count | int     |
+----------------+---------+
task_id is the primary key for this table.
Each row in this table indicates that task_id was divided into subtasks_count subtasks labelled 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 primary key 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 an SQL query to report the IDs of the missing subtasks for each task_id.

Return the result table in any order.

The query result format is in the following example:

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          |
+---------+------------+

Result table:
+---------+------------+
| task_id | subtask_id |
+---------+------------+
| 1       | 1          |
| 1       | 3          |
| 2       | 1          |
| 2       | 2          |
+---------+------------+
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.

Solution

Generate table t recursively to make sure that each task has a corresponding subtask. Then generate table cte by joining table t and table Tasks, with a column row_number added so that each task has a corresponding task_id. Finally, join cte table and Executed table amd select the entries with subtask_id is null.

# Write your MySQL query statement below
with recursive t(n) as (
    select 1 n
    union all
    select n + 1 from t where n < (select max(subtasks_count) from Tasks)
),
cte as (
    select *, row_number() over(partition by task_id) rk from Tasks, t
)
select cte.task_id,rk as subtask_id from cte left join Executed e
    on cte.task_id = e.task_id and rk = subtask_id
    where rk <= subtasks_count and subtask_id is null;

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