Question

I am a beginner when it comes to using mysql queries embedded inside other mysql queries using the IN statement.

I currently have this query:

SELECT DISTINCT BorName
FROM Borrower
WHERE BorId IN (
    SELECT Borrower.BorId
    FROM Loan
    WHERE Loan.BcId IN (
        SELECT BookCopy.BcId
        FROM BookCopy
        WHERE BookCopy.BtId In (
            SELECT BookTitle.BtId
            FROM BookTitle
            WHERE BookTitle.PubId In (
                SELECT Publisher.PubId
                FROM Publisher
                WHERE  `PubName` = CONVERT( _utf8 'Methuen' USING latin1 ) COLLATE latin1_swedish_ci
                )
            )
        )
    );

I am basically trying to find out if a borrower has borrowed a book from the publisher Methuen. I just cant seem to work out what is wrong, I have gone through each individual statement and they all seem to work just not the overall request with all of the IN statements.

Can anyone spot what is wrong?

Answer 1

Like suggested, JOINs are a much cleaner, and likely a more efficient way to do this query as opposed to nested INs:

SELECT DISTINCT b.BorName
FROM 
    Borrower b
    JOIN Loan l ON l.BorId = b.BorId
    JOIN BookCopy bc ON bc.BcId = l.BcId
    JOIN BookTitle bt ON bt.BtId = bc.BtId
    JOIN Publisher p ON p.PubId = bt.PubID
WHERE
    p.PubName = CONVERT( _utf8 'Methuen' USING latin1 ) COLLATE latin1_swedish_ci

Additionally, I think there was a problem in your first sub-query:

SELECT Borrower.BorId
FROM Loan
WHERE Loan.BcId IN...

I believe should have been:

SELECT Loan.BorId
FROM Loan
WHERE Loan.BcId IN...