How to Efficiently Detect Overlapping Time Intervals in SQL?

Efficient SQL Query for Detecting Time Interval Overlaps

One of the common challenges in database operations is to identify rows that have overlapping time intervals. This scenario is often encountered when working with schedules, booking systems, or any application that manages time-based data. A solution to this challenge involves a meticulously crafted SQL query.

Understanding the Problem

Consider two tables, both containing start and end time fields. The goal is to find, for each row in the first table (table1), all overlapping rows from the second table (table2). Overlapping intervals occur when the start time of a row in table2 falls within the time range of a row in table1, or when the end time of the row in table2 extends beyond the start time of the row in table1. It's important to note that the end time in table2 may be NULL, allowing for open-ended time intervals.

Efficient SQL Query

Oracle SQL 11g Developer Nathan Beared shared this solution, expressed as a WHERE-clause:

SELECT * 
FROM table1,table2 
WHERE table2.start <= table1.end 
AND (table2.end IS NULL OR table2.end >= table1.start)

Explanation

The key to detecting overlapping intervals is to check two conditions:

1. table2.start <= table1.end: This ensures that the start time of the interval in table2 falls within the time range of the interval in table1.
2. (table2.end IS NULL OR table2.end >= table1.start): This handles the case where the end time of the interval in table2 is either NULL (open-ended) or extends beyond the start time of the interval in table1.

By combining these conditions, the query effectively identifies rows in table2 that intersect with the time interval of each row in table1, regardless of whether table2's end time is specified or not.

This approach is efficient because it utilizes a compound index on the start and end time columns of both tables. This index enables the database to quickly retrieve the rows that satisfy the query conditions without having to perform exhaustive table scans.

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