How to Update a NULL Column in One Table Using Data from a Related Table?

Database Column Update: Matching Values Across Related Tables

Database management often requires updating a column in one table based on data from a related table. This is crucial for data integrity and bug fixes.

Scenario:

Imagine two tables: QuestionTrackings and QuestionAnswers.

• QuestionTrackings:
  • QuestionID (potentially NULL)
  • AnswerID
• QuestionAnswers:
  • AnswerID
  • QuestionID

A bug has resulted in some QuestionTrackings rows having NULL QuestionID values. Fortunately, the corresponding AnswerID values have the correct QuestionID in the QuestionAnswers table. The task is to populate the missing QuestionID values in QuestionTrackings.

Solution:

This can be efficiently accomplished using an UPDATE statement combined with an INNER JOIN:

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

• The INNER JOIN links rows with matching AnswerID values.
• SET q.QuestionID = a.QuestionID updates QuestionTrackings with the correct QuestionID from QuestionAnswers.
• WHERE q.QuestionID IS NULL limits the update to rows with missing QuestionID values.

Important Notes:

• Verification: Before executing the UPDATE statement, it's best practice to use a SELECT statement with the same JOIN to preview the results and ensure data accuracy. Confirm each AnswerID maps to a single QuestionID.
• Conditional Updates: Additional WHERE clause conditions can be added for more selective updates (e.g., WHERE q.QuestionID IS NULL AND q.Status = 'Active').

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