php loop start over when num_rows==0 - php

Question

I'm trying to write a php code to select form tables:
books
images
Some books does not have an image, so I want to skip it and select another book.
I have wrote this code but it does not work with me perfectly.
Now I'm getting only 5 records! it must be 6 as I limited in the book select query.
$slider_sql = "select * from books limit 6";
$slider_result = $conn->query($slider_sql);
while($slider_row = $slider_result->fetch_assoc()) {
extract($slider_row);
$img_sql = "SELECT big_img FROM images WHERE book_id = '$id'";
$img_rs = $conn->query($img_sql);
$img_row = $img_rs->fetch_assoc();
if ($img_rs->num_rows == 0)
continue; //--> here I want to start while again to select another book.
echo $book_name.'<br>';
echo $img_row['big_img'].'<br>';
}
Thanks for your help and time!

Answer 1

Instead of having a sub-query in a loop (which is nearly ALWAYS a bad idea!), use a JOIN instead, which simplifies it to one query instead of two. Then set a condition that big_img should not be empty. This guarantees that you will only find rows where there's an image matching the book. LIMIT will still only ensure the return of 6 rows. <> in MySQL is the same as !=.
$slider_sql = "SELECT b.book_name, i.big_img
FROM books b
JOIN images i
ON i.book_id=b.id
WHERE i.big_img <> ''
LIMIT 6";
$result = $conn->query($slider_sql);
while ($row = $result->fetch_assoc()) {
echo $row['book_name'].'<br>';
echo $row['big_img'].'<br>';
}
MySQL JOIN