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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;