Date range overlap detection in SQL

Introduction

We use date range overlap detection to find if two time periods share any common days. This helps avoid scheduling conflicts or double bookings.

Checking if two hotel room bookings overlap to prevent double booking.

Finding if employee vacation dates clash with project deadlines.

Detecting overlapping promotions or discounts in a store.

Verifying if rental car reservations overlap for the same vehicle.

Syntax

SQL

SELECT * FROM table1 t1
JOIN table2 t2 ON t1.start_date <= t2.end_date AND t1.end_date >= t2.start_date;

This syntax checks if the start of one range is before or on the end of the other, and vice versa.

It works for inclusive date ranges, meaning the start and end dates count as part of the range.

Examples

Finds all pairs of bookings that overlap, excluding comparing a booking with itself.

SQL

SELECT * FROM bookings b1
JOIN bookings b2 ON b1.start_date <= b2.end_date AND b1.end_date >= b2.start_date
WHERE b1.id != b2.id;

Detects overlapping events by comparing their start and finish dates.

SQL

SELECT * FROM events e1
JOIN events e2 ON e1.start <= e2.finish AND e1.finish >= e2.start
WHERE e1.event_id <> e2.event_id;

Sample Program

This query finds pairs of reservations that overlap by checking if their date ranges share any days. It excludes pairs where the same reservation is compared to itself and avoids duplicate pairs by using r1.id < r2.id.

SQL

CREATE TABLE reservations (
  id INT,
  start_date DATE,
  end_date DATE
);

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

SELECT r1.id AS res1, r2.id AS res2
FROM reservations r1
JOIN reservations r2 ON r1.start_date <= r2.end_date AND r1.end_date >= r2.start_date
WHERE r1.id < r2.id;

Important Notes

Make sure your date columns are stored as DATE or DATETIME types for accurate comparison.

Using '<=' and '>=' includes the boundary dates as overlapping.

To avoid duplicate pairs, use conditions like r1.id < r2.id or r1.id != r2.id carefully.

Summary

Date range overlap detection helps find if two time periods share any days.

Use conditions comparing start and end dates with <= and >= operators.

This technique is useful for scheduling, booking, and event planning.

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