How to Select Only the Most Recent Row from a Related Table in MySQL using JOINs?

SELECTing Only the Most Recent Row via MySQL JOIN

In relation to joining tables to access customer data, you may encounter situations where you want to retrieve only the most recent entry from a related table. This is prevalent in scenarios where historical data is stored. Fortunately, MySQL provides multiple approaches to achieve this.

One method involves utilizing a correlated subquery within the WHERE clause. This approach involves referencing the parent table alias to retrieve the matching row from the child table. For instance:

SELECT c.*,
FROM client AS c
LEFT JOIN client_calling_history AS cch ON cch.client_id = c.client_id
WHERE
   cch.cchid = (
      SELECT MAX(cchid)
      FROM client_calling_history
      WHERE client_id = c.client_id AND cal_event_id = c.cal_event_id
   )

This query retrieves only the most recent calling history record for each client.

Additionally, you mentioned using CONCAT within a LIKE predicate. This usage is valid in MySQL and allows you to search for substrings within concatenated column values.

In your initial query, you employed an INNER JOIN, which returns matching rows from both tables. However, you also expressed uncertainty about the appropriate JOIN type. Here's a brief explanation:

• INNER JOIN: Retrieves rows that match in both tables.
• LEFT JOIN: Retrieves all rows from the left table, even if no matches exist in the right table.
• RIGHT JOIN: Similar to LEFT JOIN, but retrieves all rows from the right table.
• FULL JOIN: Combines INNER JOIN and OUTER JOINS, returning all rows from both tables.

Choosing the correct JOIN type depends on your specific query requirements.

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