How to Efficiently Find the Earliest Record for Each Distinct ID in a Table with Duplicate Dates?

Extracting the First Record for Each Unique ID, Handling Duplicate Dates

Imagine a table (let's call it MyTable) with columns: key_id, id, record_date, and other columns (other_cols). The goal is to pinpoint the row containing the earliest record_date for every unique id.

Standard methods often assume unique record_date values, but our data might have duplicates. Complex queries using subqueries and joins can lead to unnecessary results.

A more streamlined approach uses a subquery:

<code class="language-sql">SELECT mt.*
FROM MyTable mt
INNER JOIN (
    SELECT id, MIN(record_date) AS MinDate
    FROM MyTable
    GROUP BY id
) t ON mt.id = t.id AND mt.record_date = t.MinDate;</code>

The inner subquery efficiently finds the earliest record_date (MinDate) for each id by grouping the data. The outer query then joins this result back to the original table (MyTable), selecting only the rows where the record_date matches the minimum date found in the subquery. This effectively retrieves the earliest record for each unique id, handling duplicate dates gracefully and avoiding redundant data.

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