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Question

This is how I get some BOOKs listed in my application:

$sessionid = '000000000015';

        $stmt = $conn->prepare("
SELECT 
    BOOKS_NEW.ID AS BOOKID,
    BOOKS_NEW.BOOK_NAME,
    MAX(USERS_BOOKS.DATE_MODIFIED) AS MAXDATE,
    CASE WHEN ? IN (SELECT USERID FROM USERS_LIKES WHERE BOOKID = BOOKS_NEW.ID) THEN 'Yes' ELSE 'No' END AS DID_I_LIKE_IT

FROM 
    USERS_BOOKS

INNER JOIN
    BOOKS_NEW ON BOOKS_NEW.ID=USERS_BOOKS.BOOKID



GROUP BY
    USERS_BOOKS.BOOKID

ORDER BY 
    MAXDATE DESC LIMIT $start, $finish");

    $stmt -> execute(array($sessionid));

Result

[[{"BOOKID":"000000067","BOOK_NAME":"Find a four leaf clover","MAXDATE":"2015-05-04 15:18:25","DID_I_LIKE_IT":"No"},{"BOOKID":"000000999","BOOK_NAME":"Go to a Houston Rockets game","MAXDATE":"2015-05-04 15:18:16","DID_I_LIKE_IT":"No"},{"BOOKID":"000001000","BOOK_NAME":"Go to a Houston Texans game","MAXDATE":"2015-05-04 15:18:09","DID_I_LIKE_IT":"No"}}]

However, I want to list only those BOOKs that the user ( $sessionid ) does NOT have.

This is how I list the current BOOKIDs of the user:

$stmt = $conn->prepare("SELECT USERS_BOOKS.BOOKID FROM USERS_BOOKS
    WHERE USERS_BOOKS.USERID = ?");
$stmt -> execute(array($sessionid));

RESULT:

[[{"BOOKID":"000000001"},{"BOOKID":"000000003"},{"BOOKID":"000000999"}]]

In the end result I want to get a json like the one in the first one, but without the BOOKID = 000000999 as it is present in the second one.

I tried using

WHERE NOT EXISTS (SELECT ID FROM BOOKS_NEW WHERE BOOKS_NEW.ID = USERS_BOOKS.BOOKID) 

but it gave me an empty result.

Answer 1

You have a problem in your subquery. You're referencing USERS_BOOKS.BOOKID , which is not within the scope of your subquery. What you need to do is pass the list of books already liked

WHERE BOOKS_NEW.ID NOT IN (SELECT BOOKID FROM USERS_LIKES WHERE USERID = ?)

Then pass your user ID into here. That should select all the books that the user likes and generate a list that that main query can ignore.