How to Efficiently Retrieve Records with the Highest or Lowest Values per Group in SQL?

Get Records with Highest/Smallest Values per Group

Problem:

Retrieve the records with the highest or smallest values for a given field within each group.

Former Proposed Solution:

Using ranks (@rank := @rank 1) in complex queries with subqueries.

Alternative Solution:

A more efficient approach is to utilize a left outer join without ranking variables:

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;

This query achieves the desired result by leveraging an indexing strategy on (GroupId, OrderField) to optimize access to t2.

Optimization Considerations:

• Indexing: It is critical to have a compound index on (GroupId, OrderField) for optimal performance.
• Multiple Highest/Smallest Values: To handle cases where multiple records have the same highest/smallest value within a group, extend the condition as follows:

AND (t1.OrderField < t2.OrderField 
     OR (t1.OrderField = t2.OrderField AND t1.Id < t2.Id))

Advantages of Left Outer Join Approach:

• No use of ranking or subqueries, eliminating potential performance issues.
• Better optimization, allowing the query optimizer to use indexes effectively.
• Faster execution compared to rank-based methods.

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