How to Update NULL QuestionIDs in a Database Table Using a Related Table?

Database Table Update: Aligning Columns with Referenced Values

Your question pertains to updating a database table where the QuestionID column contains null values. You seek to set these null values equal to the corresponding QuestionID values in a related table, 'Answers', referenced by the AnswerID column.

To achieve this, you can leverage an inner join. The following query demonstrates how:

UPDATE QuestionTrackings q
INNER JOIN QuestionAnswers a ON q.AnswerID = a.AnswerID
SET q.QuestionID = a.QuestionID
WHERE q.QuestionID IS NULL;

The INNER JOIN operation establishes a connection between rows in the QuestionTrackings and QuestionAnswers tables based on matching AnswerID values. This allows us to access the QuestionID value from the QuestionAnswers table for each row in the QuestionTrackings table.

You can add additional conditions to the WHERE clause to further narrow down the rows to be updated. For example, if you only want to update rows where QuestionID is null and AnswerID is less than 500:

UPDATE QuestionTrackings q
INNER JOIN QuestionAnswers a ON q.AnswerID = a.AnswerID
SET q.QuestionID = a.QuestionID
WHERE q.QuestionID IS NULL AND q.AnswerID <p>It's recommended to preview the rows to be updated using a select query before executing the update to ensure accuracy:</p><pre class="brush:php;toolbar:false">SELECT *
FROM QuestionTrackings q
INNER JOIN QuestionAnswers a ON q.AnswerID = a.AnswerID
WHERE q.QuestionID IS NULL;

This will help you verify that each AnswerID has only one associated QuestionID and avoid potential errors during the update process.

The above is the detailed content of How to Update NULL QuestionIDs in a Database Table Using a Related Table?. For more information, please follow other related articles on the PHP Chinese website!