Question

我在mysql中有3个表1个用户，2个问题和3个结果。

我写了以下查询。

SELECT q.id,qr.user_id 
FROM questions AS q 
    LEFT JOIN results AS r ON q.id=r.question_id 
        and r.user_id in (SELECT user_id FROM students)

如果该问题的结果中不存在答案。我想要结果中的问题和用户ID。例子

user
U1
U2
U3

question
A
B
C

result
A U1 YES
A U2 YES
A U3 YES
B U1 YES
B U2 YES
B U3 YES
C U2 YES
C U3 YES

Desired Output
A U1 YES
A U2 YES
A U3 YES
B U1 YES
B U2 YES
B U3 YES
**C U1 No**
C U2 YES
C U3 YES

请帮助。

Answer 1

尝试一下

add(book: Book) {
        return this.authentication.user.pipe(
            take(1),
            switchMap(user => {
                // Set owner for the backend to handle correctly
                book.setOwner(user.uid);

                // Add book request
                const queueRef = this._afqueue.list(this.ADD_BOOK_QUEUE_PATH);
                const pushPromise = queueRef.push({ status: { code: 'NEW' }, ...book })
                    .then(ref => {
                        console.log('Request to add a new book added to queue.');
                        return ref;
                    }) as Promise<any>;
                return from(pushPromise);
            }),
            switchMap(ref => {
                return this._afqueue.object(this.ADD_BOOK_QUEUE_PATH + '/' + ref.key)
                    .valueChanges();
            }),
            map(snap => snap['status']),
            filter(status => status['code'] === 'SUCCESS' || status['code'] === 'ERROR'),
            switchMap(status => {
                if (status['code'] === 'SUCCESS') {
                    return Observable.create(function(observer) {
                        observer.complete(status['book_id']);
                      });
                    //return status['book_id'];
                }
                else if (status['code'] === 'ERROR') {
                    throw(status['error']);
                }
            }),
            timeout(60000), // timeout after 60 secondes
        );
    }

要在左联接选择中匹配ID的值

1 个答案: