How to Efficiently Find Players on a Team Within a Specific Date Range in PostgreSQL?

Finding Overlapping Date Ranges in PostgreSQL

The task at hand is to retrieve a list of players associated with a specific team within a designated time frame. While the provided query attempts to achieve this, it falls short due to incorrect logic and a fundamental misunderstanding of how to identify overlapping ranges.

To create a query that accurately identifies players within the desired time frame, several key principles must be considered:

1. Proper Date Range Comparison: The BETWEEN operator, as used in the original query, includes the upper bound. This is incorrect when dealing with overlapping ranges, which should be exclusive. Instead, the appropriate comparison is a < or = between the lower bound and upper bound, ensuring that only dates within the specified range are captured.
2. Consideration of NULL Values: For those players who have not yet left the team, the date_leave column may be NULL. The query must account for this possibility by allowing for NULL values to be included in the overlapping range comparison.
3. Handling Potential Duplicates: When multiple contracts exist for the same player, it's imperative to consolidate the results to eliminate duplicates. The DISTINCT keyword can be employed to ensure that only unique player names are returned.
4. SQL OVERLAPS Operator: PostgreSQL offers a built-in OVERLAPS operator, which is specifically designed to determine whether two time periods intersect. This operator can simplify the query by eliminating the need for explicit date comparisons.
5. Range Types: In PostgreSQL 9.2 and later, specialized range types are available for representing date ranges. These types allow for efficient overlapping range comparisons and can be utilized to optimize the query.

By incorporating these principles, the revised query provided addresses the shortcomings of the original query and accurately identifies players of a given team within the specified time frame:

SELECT DISTINCT name_player 
FROM   contract
WHERE  name_team = ? 
AND    (date_join, COALESCE(date_leave, CURRENT_DATE)) OVERLAPS
       (date '2009-01-01', date '2010-01-01');

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