How to Identify Overlapping Date Ranges in PostgreSQL for Player Team Membership?

Identifying Overlapping Date Ranges in PostgreSQL

Problem

A user seeks to retrieve a list of players who belonged to a specific team within a given year range. The initially proposed query is as follows:

SELECT * 
FROM   contract 
JOIN   team USING (name_team) 
JOIN   player USING(name_player) 
WHERE  name_team = ? 
AND    DATE_PART('YEAR',date_join)>= ? 
AND    DATE_PART('YEAR',date_leave)<= ?

However, this query does not effectively identify overlapping date ranges.

Correct Solution

To accurately determine overlapping date ranges, the query must be modified to consider the following criteria:

1. Players who joined the team before the start of the specified year range.
2. Players who did not leave the team before the end of the specified year range.
3. Players who never left the team (represented by NULL date_leave values).

Optimized Query

The following query provides a more accurate solution:

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');  -- upper bound excluded

Explanation

• The COALESCE function is used to handle NULL date_leave values, considering them as open ranges.
• The OVERLAPS operator automatically sets the start of each interval as the earlier value of the pair.
• The daterange '[2009-01-01,2010-01-01)' represents a half-open interval, excluding the upper bound.

Additional Considerations

• In PostgreSQL 9.2 or later, range types can be used with index support, providing improved performance for range-based queries.
• The provided query can be further optimized by creating an expression index on the daterange(date_join, date_leave) expression.

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