从2个表中选择并搜索值-selectfrom2tablesandsearchforavalue

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This is what you can do to get the number of rows returned,

这是你可以做的，以获得返回的行数，

while ($book = mysqli_fetch_assoc($results)) {
    $bokId = $book['id'];

    // Prepare query
    $checkBorrow = $db->prepare('SELECT * FROM bookingbook LEFT JOIN borrowbook ON 1=1 WHERE bookingbook.bookId=? OR borrowbook.bookId=? UNION SELECT * FROM bookingbook RIGHT JOIN borrowbook ON 1=1 WHERE bookingbook.bookId=? OR borrowbook.bookId=?');

    // Binds variables to prepared statement
    $checkBorrow->bind_param('iiii', $bokId, $bokId, $bokId, $bokId);

    // Execute query
    $checkBorrow->execute();

    // Store the result (to get properties)
    $checkBorrow->store_result();

    // Get the number of rows
    $numRows = $checkBorrow->num_rows;

    if ($numRows) {
        $states = "Not available";
    } else{
        $states = "Available";
    }

    // free results
    $checkBorrow->free_result();

    // close statement
    $checkBorrow->close();
}

Caution: You're mixing the procedural and object oriented style of mysqli. In the while loop condition you're using procedural style, whereas inside the while loop block you're using object oriented style of mysqli. Please pick one style and stick to it.

警告：你正在混合mysqli的程序和面向对象的风格。在while循环条件中，您使用的是过程样式，而在while循环块中，您使用的是面向对象的mysqli样式。请选择一种风格并坚持下去。

Edited:

编辑：

Updated SQL query

更新了SQL查询

// Prepare query
$checkBorrow = $db->prepare('
SELECT * FROM bookingbook LEFT JOIN borrowbook ON 1=1 WHERE bookingbook.bookId=? OR borrowbook.bookId=? 
UNION 
SELECT * FROM bookingbook RIGHT JOIN borrowbook ON 1=1 WHERE bookingbook.bookId=? OR borrowbook.bookId=?');

// Binds variables to prepared statement
$checkBorrow->bind_param('iiii', $bokId, $bokId, $bokId, $bokId);