How Can I Efficiently Retrieve the Highest or Lowest Record per Group in a Database?

Retrieving Records with Highest/Lowest Per Group

Problem Statement

How can one efficiently retrieve the record with the highest or lowest value for a specific field within each group defined by another field?

Solution Using Left Outer Join

To retrieve the record with the highest value for the OrderField field within each GroupId, use the following query:

SELECT t1.*
FROM `Table` AS t1
LEFT OUTER JOIN `Table` AS t2
  ON t1.GroupId = t2.GroupId AND t1.OrderField < t2.OrderField
WHERE t2.GroupId IS NULL
ORDER BY t1.OrderField;

Benefits of Join Approach

• Efficient for large datasets
• Uses an index on (GroupId, OrderField) if available
• No need for subqueries or sorting within the query

Alternatives

If the data is small or the specific implementation of the database engine you are using is poorly optimized for joins, you may consider alternatives such as:

• Using a Subquery: This involves using a subquery to calculate the maximum value for each group and then joining this subquery to the main table to retrieve the corresponding records.
• Using Rank Function: This involves using a window function to calculate the rank of each record within each group and then filtering based on the rank.

Inefficiency of Method Using @Rank Variable

The method you described using the @Rank variable, as originally written, will not work correctly because the @Rank variable does not reset to zero after processing the first table. To fix this, you would need to add another derived table to reset @Rank to zero before processing the second table. However, this approach is still inefficient compared to the join approach.

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