How to Update NULL QuestionIDs in a Tracking Table Using Related AnswerIDs?

Updating NULL Question IDs Using Associated Answer IDs

Challenge:

A tracking table contains rows with NULL values in the "QuestionID" column. Fortunately, the related "AnswerID" column in the same table holds the key to finding the correct "QuestionID" within the "Answers" table. The goal is to populate these NULL "QuestionID" entries with the corresponding values from the "Answers" table.

Solution:

Here's the SQL query to accomplish this update:

<code class="language-sql">UPDATE QuestionTrackings q
INNER JOIN QuestionAnswers a ON q.AnswerID = a.AnswerID
SET q.QuestionID = a.QuestionID
WHERE q.QuestionID IS NULL;</code>

Explanation:

This query uses an INNER JOIN to connect the QuestionTrackings (aliased as q) and QuestionAnswers (aliased as a) tables. The join condition q.AnswerID = a.AnswerID ensures that only matching rows (based on AnswerID) are considered. The SET clause assigns the a.QuestionID value to q.QuestionID for each row where q.QuestionID is NULL.

Important Considerations:

• Preview the Update: Before running the UPDATE statement, execute a SELECT query (using the same JOIN and WHERE clauses) to preview the rows that will be affected. This allows you to verify the accuracy of the update before making any changes to your data.

• Data Integrity: Confirm that each AnswerID in the QuestionTrackings table uniquely maps to a single QuestionID in the QuestionAnswers table. Duplicate AnswerID values could lead to unpredictable update results.

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