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Report Contiguous Dates - Problem

Database Hard

You're tasked with analyzing the historical performance of a daily task system running throughout 2019. The system executes exactly one independent task each day, which either succeeds or fails.

Given two tables:

Failed - Contains dates when tasks failed
Succeeded - Contains dates when tasks succeeded

Your goal: Generate a report showing contiguous periods of consecutive days with the same outcome (all failed or all succeeded) from 2019-01-01 to 2019-12-31.

Each period should include:

period_state: Either 'failed' or 'succeeded'
start_date: First date of the consecutive period
end_date: Last date of the consecutive period

Example: If tasks failed on Jan 1-3, succeeded on Jan 4-7, then failed on Jan 8, you'd have three periods: failed (Jan 1-3), succeeded (Jan 4-7), and failed (Jan 8-8).

Input & Output

basic_example.sql — Basic Case

$ Input: Failed table: [2019-01-01, 2019-01-02, 2019-01-03] Succeeded table: [2019-01-04, 2019-01-05]

› Output: start_date: 2019-01-01, end_date: 2019-01-03, period_state: failed start_date: 2019-01-04, end_date: 2019-01-05, period_state: succeeded

💡 Note: First 3 days failed consecutively, then 2 days succeeded consecutively, creating two distinct periods

alternating_example.sql — Alternating Status

$ Input: Failed table: [2019-01-01, 2019-01-03, 2019-01-05] Succeeded table: [2019-01-02, 2019-01-04]

› Output: start_date: 2019-01-01, end_date: 2019-01-01, period_state: failed start_date: 2019-01-02, end_date: 2019-01-02, period_state: succeeded start_date: 2019-01-03, end_date: 2019-01-03, period_state: failed start_date: 2019-01-04, end_date: 2019-01-04, period_state: succeeded start_date: 2019-01-05, end_date: 2019-01-05, period_state: failed

💡 Note: Status alternates daily, so each day forms its own single-day period

year_boundary.sql — Year Boundary

$ Input: Failed table: [2019-12-30, 2019-12-31] Succeeded table: [2019-12-29]

› Output: start_date: 2019-12-29, end_date: 2019-12-29, period_state: succeeded start_date: 2019-12-30, end_date: 2019-12-31, period_state: failed

💡 Note: Tests the year boundary constraint - only dates within 2019 are considered, forming one success period and one failure period

Constraints

fail_date and success_date are unique within their respective tables
All dates are within the range 2019-01-01 to 2019-12-31
The system runs exactly one task per day (no missing days in the union)
A date cannot appear in both Failed and Succeeded tables
Result must be ordered by start_date ascending

Visualization

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Understanding the Visualization

Collect All Reports

Gather all success and failure dates from 2019, like collecting daily reports from two different departments

Sort Chronologically

Arrange all reports in date order, mixing successes and failures in chronological sequence

Number Within Groups

Assign sequential numbers within each status type (1st success, 2nd success, etc.)

Calculate Grouping Key

Subtract group numbers from overall sequence - this difference stays constant within consecutive periods

Bundle Consecutive Periods

Group by the calculated key to bundle consecutive days of same status together

Key Takeaway

🎯 Key Insight: Using ROW_NUMBER() with the difference technique elegantly groups consecutive dates by exploiting the mathematical property that rank differences remain constant within contiguous sequences of the same status.

Asked in

G Google 45 a Amazon 38 ⊞ Microsoft 32 f Meta 28

The optimal solution uses window functions with row numbering to detect consecutive sequences. By calculating the difference between overall rank and status-specific rank, we can group consecutive dates with the same status. This O(n log n) approach is elegant and leverages SQL's built-in analytical capabilities effectively.

Common Approaches

Approach	Time	Space	Notes
✓ Brute Force (Date Range Scanning)	O(n³)	O(n)	Check every possible date range to identify contiguous periods
Two-Pass Status Change Detection	O(n log n)	O(n)	First pass to collect all dates, second pass to detect status changes
Window Function with Row Numbering (Optimal)	O(n log n)	O(n)	Use ROW_NUMBER() to detect consecutive sequences in a single pass

Brute Force (Date Range Scanning) — Algorithm Steps

Generate all dates from 2019-01-01 to 2019-12-31
For each possible start date, try all possible end dates
Check if the date range contains only failed or only succeeded tasks
Record valid contiguous periods

Visualization

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Step-by-Step Walkthrough

Generate All Ranges

Create every possible start-end date combination

Check Consistency

For each range, verify all dates have same status

Validate Boundaries

Ensure range cannot be extended further

Collect Results

Store valid contiguous periods

Code -

solution.c — C

// Brute force SQL approach
#include <stdio.h>
#include <string.h>

char* solution() {
    static char query[] = 
    "WITH date_sequence AS (\n"
    "    SELECT DATE('2019-01-01') + INTERVAL (num) DAY as date_val\n"
    "    FROM (SELECT 0 as num UNION SELECT 1 UNION SELECT 2) numbers\n"
    "    WHERE DATE('2019-01-01') + INTERVAL (num) DAY <= '2019-12-31'\n"
    "),\n"
    "status_dates AS (\n"
    "    SELECT d.date_val,\n"
    "           CASE WHEN f.fail_date IS NOT NULL THEN 'failed'\n"
    "                WHEN s.success_date IS NOT NULL THEN 'succeeded'\n"
    "                END as status\n"
    "    FROM date_sequence d\n"
    "    LEFT JOIN Failed f ON d.date_val = f.fail_date\n"
    "    LEFT JOIN Succeeded s ON d.date_val = s.success_date\n"
    ")\n"
    "SELECT MIN(date_val) as start_date,\n"
    "       MAX(date_val) as end_date,\n"
    "       status as period_state\n"
    "FROM status_dates\n"
    "GROUP BY status\n"
    "ORDER BY start_date";
    
    return query;
}

Time & Space Complexity

Time Complexity

⏱️

O(n³)

For each of n² possible date ranges, we check n dates to verify consistency

⚠ Quadratic Growth

Space Complexity

O(n)

Space to store all dates and their statuses

⚡ Linearithmic Space

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Brute Force (Date Range Scanning) — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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