Update columns with values ​​from related tables

This article explores how to update a column in a table (QuestionTrackings) whose values ​​come from a related table (QuestionAnswers) based on a specific condition (QuestionID is NULL). We can achieve this using UPDATE statement combined with INNER JOIN.

SQL query:

UPDATE QuestionTrackings q
INNER JOIN QuestionAnswers a
ON q.AnswerID = a.AnswerID
SET q.QuestionID = a.QuestionID
WHERE q.QuestionID IS NULL
-- 可在此处添加其他条件

illustrate:

• Use INNER JOIN to establish a join between two tables based on the common column AnswerID.
• Condition q.QuestionID IS NULL Ensures that updates only affect rows in the QuestionTrackings table where QuestionID is NULL.
The
• SET clause updates the QuestionID column in the QuestionTrackings table (q.QuestionID) with the corresponding QuestionID from the QuestionAnswers table (a.QuestionID).

Note:

Before executing an UPDATE query, it is recommended to preview the affected data. To do this, you can modify the query to include the following:

SELECT *
FROM QuestionTrackings q
INNER JOIN QuestionAnswers a
ON q.AnswerID = a.AnswerID
WHERE q.QuestionID IS NULL
-- 可在此处添加其他条件

This will display the rows that will be updated and confirm that each AnswerID has only one associated QuestionID to prevent data inconsistencies.