13. Repeated Values in Sequence (Medium)

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13. Repeated Values in Sequence (Medium)

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Problem Statement

Table: Sequence

Each row in this table represents a sequence of numbers with a unique ID and a value for each position in the sequence.

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| id          | int     |
| value       | int     |
+-------------+---------+
id is the primary key for this table.
This table may contain duplicate values in the 'value' column.

Develop a solution to find all values that appear at least three times consecutively in the sequence.

Return the result table in any order.

Example

Input:

Sequence table:
+----+-------+
| id | value |
+----+-------+
| 1  | 100   |
| 2  | 100   |
| 3  | 100   |
| 4  | 200   |
| 5  | 100   |
| 6  | 200   |
| 7  | 200   |
+----+-------+

Output:

+-----------------+
| RepeatedValues  |
+-----------------+
| 100             |
+-----------------+

In this example, 100 is the only value that appears consecutively for at least three times.

Try It Yourself

MYSQL

. . . .

Solution

To identify all values that appear at least three times consecutively in the Sequence table, we can utilize SQL window functions such as LAG and LEAD. This approach allows us to examine adjacent rows efficiently and determine if a value repeats consecutively across the desired number of positions.

Approach Overview

Use Window Functions to Access Adjacent Rows: Employ LAG to retrieve the previous value and LEAD to retrieve the next value for each row.
Identify Values with Three Consecutive Repetitions: Select values where the current value matches both the previous and next values, indicating three consecutive occurrences.
Eliminate Duplicate Results: Use DISTINCT to ensure each repeated value appears only once in the final output.

SQL Query

WITH Consecutive AS (
    SELECT value,
           LAG(value, 1) OVER (ORDER BY id) AS prev_value,
           LEAD(value, 1) OVER (ORDER BY id) AS next_value
    FROM Sequence
)
SELECT DISTINCT value AS RepeatedValues
FROM Consecutive
WHERE value = prev_value AND value = next_value;

Step-by-Step Approach

Step 1: Use Window Functions to Access Adjacent Rows

Retrieve each value along with its immediate predecessor and successor to identify potential consecutive repetitions.

SQL Query:

WITH Consecutive AS (
    SELECT 
        value,
        LAG(value, 1) OVER (ORDER BY id) AS prev_value,
        LEAD(value, 1) OVER (ORDER BY id) AS next_value
    FROM 
        Sequence
)

Explanation:

WITH Consecutive AS (...):
- Defines a Common Table Expression (CTE) named Consecutive that prepares the data for further analysis.
SELECT value, LAG(value, 1) OVER (ORDER BY id) AS prev_value, LEAD(value, 1) OVER (ORDER BY id) AS next_value:
- value: The current value in the sequence.
- LAG(value, 1) OVER (ORDER BY id) AS prev_value: Retrieves the value from the previous row based on the id ordering.
- LEAD(value, 1) OVER (ORDER BY id) AS next_value: Retrieves the value from the next row based on the id ordering.
FROM Sequence:
- Indicates that the data is sourced from the Sequence table.

Output After Step 1:

+-------+------------+------------+
| value | prev_value | next_value |
+-------+------------+------------+
| 100   | NULL       | 100        |
| 100   | 100        | 100        |
| 100   | 100        | 200        |
| 200   | 100        | 100        |
| 100   | 200        | 200        |
| 200   | 100        | 200        |
| 200   | 200        | NULL       |
+-------+------------+------------+

Step 2: Identify Values with Three Consecutive Repetitions

Filter the records to find values that are the same as both their preceding and succeeding values, indicating three consecutive occurrences.

SQL Query:

SELECT DISTINCT value AS RepeatedValues
FROM Consecutive
WHERE value = prev_value AND value = next_value;

Explanation:

SELECT DISTINCT value AS RepeatedValues:
- Selects unique values that meet the repetition criteria and aliases the column as RepeatedValues.
FROM Consecutive:
- Utilizes the Consecutive CTE defined in Step 1.
WHERE value = prev_value AND value = next_value:
- Filters rows where the current value is equal to both prev_value and next_value, ensuring that the value appears consecutively at least three times.

Output After Step 2:

+-----------------+
| RepeatedValues  |
+-----------------+
| 100             |
+-----------------+

.....

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