How to Efficiently Retrieve the Most Recent Historic Location for a Given Record ID Using SQL?

Retrieving Historic Locations Using MAX Date within a Join Statement

When fetching historical locations for a specific record ID, a user encountered performance issues and duplicate entries in the results. The user attempted to address this by joining multiple tables and filtering by the received_id, but the expected output was not achieved.

To resolve this, it is recommended to modify the query as follows:

SELECT t1.received_id
     , t1.transaction_id
     , t1.date_modified
     , l.location
  FROM transactions t1
  JOIN ( SELECT received_id, MAX(date_modified) maxmodify FROM transactions GROUP BY received_id) max_record
    ON max_record.received_id = t1.received_id 
   AND max_record.maxmodify = t1.date_modified
  JOIN locations l
    ON l.location_id = t1.location_id
  JOIN received r
    ON r.received_id = t1.received_id
 WHERE t1.received_id = '1782'
 ORDER 
    BY t1.date_modified DESC

The key change involves joining the transactions table twice: Once for the main query and once within a subquery to find the maximum date_modified value for each received_id. This subquery is then used to filter the main query and return only the rows with the latest date_modified values.

The core of this technique is encapsulated in the following general pattern:

SELECT x.*
  FROM my_table x
  JOIN (SELECT id,MAX(thing) max_thing FROM my_table GROUP BY id) y 
    ON y.id = x.id 
   AND y.max_thing = x.thing;

This approach effectively eliminates duplicate entries and provides the desired result of displaying only the latest modified time for each record ID and location.

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