How to Select Only the Most Recent Location Data Within a SQL Join?-Mysql Tutorial-php.cn

How to Select Only the Most Recent Location Data Within a SQL Join?

Patricia Arquette

Release： 2024-12-04 17:47:11

Original

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How to Select Only the Most Recent Location Data Within a SQL Join?

Selecting Maximum Date Within a Join Operation

To retrieve historic locations for a specific record, consider the following query:

SELECT l.location, t.transaction_id, t.date_modified
FROM transactions AS t
INNER JOIN (
  SELECT
    received_id,
    MAX(date_modified) AS maxmodify
  FROM transactions
  GROUP BY
    received_id
) AS max_record
  ON (
    t1.received_id = max_record.received_id
  )
INNER JOIN locations AS l
  ON (
    l.location_id = t.location_id
  )
INNER JOIN received AS r
  ON (
    r.received_id = t.received_id
  )
WHERE
  t.received_id = '1782'
ORDER BY
  t.date_modified DESC;

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This query returns a list of locations for a given record, but it includes multiple entries for the same location and date. To ensure only the last modified date is displayed for each location, use the following query:

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

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The key modification is the addition of the join condition that ensures that the date_modified value selected is the maximum for each record. This ensures that only the most recently updated location information is displayed.

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