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To ensure that every data entry per ID is retained on JOINED tables under a specific condition, you can use the following steps:

1. Identify the tables that contain the data entries per ID, and the condition that needs to be met for retention.

2. Use the INNER JOIN statement to combine the tables based on the relevant common column(s) that contain the ID.

3. Next, use the WHERE clause to specify the condition that ensures the data entry for each ID is retained. For example, you can use the WHERE clause to filter the results based on a specific date range or a certain value in a column.

4. Finally, use the GROUP BY statement to group the data entries by the ID column, ensuring that all entries for each ID are retained.

An example SQL query that implements the above steps would look like this:

SELECT * FROM table1 INNER JOIN table2 ON table1.ID = table2.ID WHERE table1.date BETWEEN 'startdate' AND 'enddate' GROUP BY table1.ID

This query ensures that every data entry per ID is retained on JOINED tables if it meets the condition of falling within a particular date range.