Question

ALTER TABLE Question ADD CONSTRAINT FK_SessionQuestion FOREIGN KEY
(SessionId) REFERENCES Session (SessionId);

SQL above gives me error below:

#1452 – Cannot add or update a child row: a foreign key constraint fails (mobile_app.#sql-567_20be3, CONSTRAINT FK_SessionQuestion
FOREIGN KEY (SessionId) REFERENCES Session (SessionId))

I want to include a foreign key in Question table to link up with SessionId but why won’t it let me and how can it be fixed?

Question table:

SessionId(PK) QuestionId(PK) QuestionContent
--------------------------------------------
1             1              What is 2+2
1             2              What is 3+3
2             1              What is 5+5
2             2              What is 4+7

Session table:

SessionId SessionName
---------------------
1         AAA

Answer 1

The question table includes SessionIds that are not found in the session table (well, actually, just one: 2). The FOREIGN KEY constraint, however, requires every SessionId used in the question table to exist in the session table by its definition.

Either insert all missing sessions, or remove all questions that reference sessions that don’t exist.

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To get a list of all SessionIds you’re missing in the table Session, you can use a LEFT JOIN:

SELECT DISTINCT Question.SessionId
FROM Question
     LEFT JOIN Session
       ON Question.SessionId = Session.SessionId
WHERE Session.SessionId IS NULL;