Date range overlap detection in SQL - Mini Project: Build & Apply

Date Range Overlap Detection

📖 Scenario: You are managing a booking system for a small hotel. Each booking has a start date and an end date. You want to find if any two bookings overlap in their date ranges to avoid double bookings.

🎯 Goal: Build a SQL query that detects overlapping date ranges between bookings in the database.

📋 What You'll Learn

Create a table called bookings with columns id, start_date, and end_date.

Insert sample booking data with specific date ranges.

Write a query to find pairs of bookings where the date ranges overlap.

Ensure the query excludes comparing a booking with itself.

💡 Why This Matters

🌍 Real World

Detecting overlapping date ranges is essential in booking systems, event scheduling, and resource allocation to avoid conflicts.

💼 Career

Database developers and analysts often write queries to find overlapping intervals to ensure data integrity and proper scheduling.

1

Create the bookings table and insert data

Create a table called bookings with columns id as integer primary key, start_date as date, and end_date as date. Then insert these exact rows: (1, '2024-06-01', '2024-06-05'), (2, '2024-06-04', '2024-06-10'), (3, '2024-06-11', '2024-06-15').

SQL

-- Create the bookings table and insert the specified rows
-- Your code here

Hint

Use CREATE TABLE to define the table and INSERT INTO to add rows.

2

Add a helper condition for overlap detection

Create a variable or expression that will help detect if two date ranges overlap. Use the condition that two ranges overlap if the start date of one is less than or equal to the end date of the other and vice versa. Write this condition as part of a WHERE clause comparing b1 and b2 bookings.

SQL

CREATE TABLE bookings (
  id INTEGER PRIMARY KEY,
  start_date DATE,
  end_date DATE
);

INSERT INTO bookings (id, start_date, end_date) VALUES
  (1, '2024-06-01', '2024-06-05'),
  (2, '2024-06-04', '2024-06-10'),
  (3, '2024-06-11', '2024-06-15');

-- Add a WHERE clause condition to detect overlap between b1 and b2
-- Your code here

Hint

Two date ranges overlap if one starts before the other ends and ends after the other starts.

3

Write the core query to find overlapping bookings

Write a SQL query that selects b1.id and b2.id from the bookings table aliased as b1 and b2. Use the overlap condition from Step 2 in the WHERE clause. Also, add a condition to exclude rows where b1.id = b2.id to avoid comparing a booking with itself.

SQL

CREATE TABLE bookings (
  id INTEGER PRIMARY KEY,
  start_date DATE,
  end_date DATE
);

INSERT INTO bookings (id, start_date, end_date) VALUES
  (1, '2024-06-01', '2024-06-05'),
  (2, '2024-06-04', '2024-06-10'),
  (3, '2024-06-11', '2024-06-15');

SELECT b1.id, b2.id
FROM bookings b1, bookings b2
WHERE b1.id  b2.id
  AND b1.start_date = b2.start_date
-- Your code here

Hint

Use a self-join on the bookings table with aliases and apply the overlap condition and id inequality.

4

Complete the query with ordering and distinct pairs

Modify the query to select distinct pairs of overlapping bookings by adding DISTINCT before the select columns. Also, order the results by b1.id ascending and then b2.id ascending.

SQL

CREATE TABLE bookings (
  id INTEGER PRIMARY KEY,
  start_date DATE,
  end_date DATE
);

INSERT INTO bookings (id, start_date, end_date) VALUES
  (1, '2024-06-01', '2024-06-05'),
  (2, '2024-06-04', '2024-06-10'),
  (3, '2024-06-11', '2024-06-15');

-- Add DISTINCT and ORDER BY clauses to the query
-- Your code here

Hint

Use DISTINCT to avoid duplicate pairs and ORDER BY to sort the results.

Practice

1. What does date range overlap detection in SQL help you find?
SELECT * FROM bookings WHERE start_date <= '2024-05-10' AND end_date >= '2024-05-05';

easy

A. If dates are in the future only

B. If two date ranges share any common days

C. If dates are stored as strings

D. If one date range is completely before another

Solution

1. Step 1: Understand the condition

   The query checks if the booking's start date is on or before May 10 and the end date is on or after May 5.

2. Step 2: Interpret the overlap logic

   This means the booking period overlaps with the date range May 5 to May 10.

3. Final Answer:

   If two date ranges share any common days -> Option B

4. Quick Check:

   Date overlap means shared days = D [OK]

Hint: Check if start <= other end and end >= other start [OK]

Common Mistakes:

• Thinking overlap means one range is inside another
• Confusing date comparison operators
• Ignoring equal dates as overlap

2. Which SQL WHERE clause correctly detects overlap between two date ranges (start1, end1) and (start2, end2)?

easy

A. WHERE start1 < end2 AND end1 > start2

B. WHERE start1 > end2 OR end1 < start2

C. WHERE start1 = end2 AND end1 = start2

D. WHERE start1 <= end2 AND end1 >= start2

Solution

1. Step 1: Recall overlap condition

   Two ranges overlap if the start of one is on or before the end of the other, and the end of one is on or after the start of the other.

2. Step 2: Match condition to options

   WHERE start1 <= end2 AND end1 >= start2 uses <= and >= which correctly includes touching dates as overlap.

3. Final Answer:

   WHERE start1 <= end2 AND end1 >= start2 -> Option D

4. Quick Check:

   Overlap needs <= and >=, not just < or > [OK]

Hint: Use <= and >= to include touching dates as overlap [OK]

Common Mistakes:

• Using < and > excludes touching dates
• Using OR instead of AND
• Checking equality only

3. Given the table events(event_id, start_date, end_date) with data:
1, '2024-05-01', '2024-05-05'
2, '2024-05-04', '2024-05-10'
3, '2024-05-11', '2024-05-15'
What rows will this query return?
SELECT * FROM events WHERE start_date <= '2024-05-06' AND end_date >= '2024-05-04';

medium

A. Rows with event_id 1 and 2

B. Rows with event_id 2 and 3

C. Only row with event_id 2

D. All rows

Solution

1. Step 1: Check each event against the condition

   Condition: start_date <= '2024-05-06' AND end_date >= '2024-05-04'.
   Event 1: start '2024-05-01' <= '2024-05-06' (true), end '2024-05-05' >= '2024-05-04' (true) -> included.
   Event 2: start '2024-05-04' <= '2024-05-06' (true), end '2024-05-10' >= '2024-05-04' (true) -> included.
   Event 3: start '2024-05-11' <= '2024-05-06' (false) -> excluded.

2. Step 2: Determine final result set

   Only events 1 and 2 satisfy the overlap condition.

3. Final Answer:

   Rows with event_id 1 and 2 -> Option A

4. Quick Check:

   Events overlapping May 4-6 are 1 and 2 [OK]

Hint: Test each row's start and end against the range [OK]

Common Mistakes:

• Including event 3 which starts after the range
• Ignoring the >= condition on end_date
• Assuming only exact matches count

4. Identify the error in this SQL query intended to find overlapping bookings:
SELECT * FROM bookings WHERE start_date > '2024-05-01' AND end_date < '2024-05-10';

medium

A. All of the above

B. It uses > and < instead of >= and <=, missing overlaps

C. It only finds bookings fully inside the range, not overlapping ones

D. It excludes bookings that start or end exactly on the boundary dates

Solution

1. Step 1: Analyze the condition logic

   The query uses start_date > '2024-05-01' and end_date < '2024-05-10', which means it only selects bookings fully inside the range, excluding those starting or ending exactly on the boundaries.

2. Step 2: Understand overlap detection requirements

   Overlap detection requires using <= and >= to include touching dates and also to detect partial overlaps, not just fully contained bookings.

3. Step 3: Combine observations

   All options B, C, and D are true, so the query is incorrect for overlap detection.

4. Final Answer:

   All of the above -> Option A

5. Quick Check:

   All B, C, D are errors = A [OK]

Hint: Use inclusive operators and allow partial overlaps [OK]

Common Mistakes:

• Using strict > and < excludes boundary overlaps
• Assuming fully inside means overlap
• Not testing edge cases

5. You have a table reservations(id, start_date, end_date). Write a query to find all reservations that overlap with a new booking from '2024-06-10' to '2024-06-15'. Which query correctly detects overlaps including edge cases?

hard

A. SELECT * FROM reservations WHERE start_date < '2024-06-15' AND end_date > '2024-06-10';

B. SELECT * FROM reservations WHERE start_date BETWEEN '2024-06-10' AND '2024-06-15';

C. SELECT * FROM reservations WHERE start_date <= '2024-06-15' AND end_date >= '2024-06-10';

D. SELECT * FROM reservations WHERE end_date BETWEEN '2024-06-10' AND '2024-06-15';

Solution

1. Step 1: Understand overlap with inclusive edges

   Overlap means any shared day, including when one reservation ends exactly when the new booking starts or vice versa.

2. Step 2: Evaluate each option

   SELECT * FROM reservations WHERE start_date < '2024-06-15' AND end_date > '2024-06-10'; uses < and > which excludes exact boundary matches.
   SELECT * FROM reservations WHERE start_date <= '2024-06-15' AND end_date >= '2024-06-10'; uses <= and >= which includes edge overlaps.
   Options C and D only check if start or end is inside the new booking range, missing some overlaps.

3. Step 3: Choose the correct query

   SELECT * FROM reservations WHERE start_date <= '2024-06-15' AND end_date >= '2024-06-10'; correctly detects all overlaps including edge cases.

4. Final Answer:

   SELECT * FROM reservations WHERE start_date <= '2024-06-15' AND end_date >= '2024-06-10'; -> Option C

5. Quick Check:

   Inclusive operators catch all overlaps = B [OK]

Hint: Use <= and >= to catch all overlaps including edges [OK]

Common Mistakes:

• Using < and > excludes edge overlaps
• Using BETWEEN misses partial overlaps
• Checking only start or end date