How to limit MySQL query result to specific results? - php

Question

I have a news table in MySQL Database I query the table and show the title of the news in one PHP pages but now the table getting bigger so I want to divide the results into pages I mean to show each 50 news title in one page (pagenation).
I use this query to bring the news :
SELECT news.*, categories.category, users.username
FROM news INNER JOIN
users on news.user_id = users.id INNER JOIN
categories on categories.id = news.category_id
order by news.timestamp DESC
limit $min,$max
and this is part of the PHP page (How I calculate the max and min)
$news_per_page = 50;
if(!empty($_GET['p_n']))
{
$p_n = $_GET['p_n'];
$max = $news_per_page*$p_n;
$min = ($max - $news_per_page);
}
else
{
$p_n = 1;
$max = 50;
$min = 0;
}
$news = all_news($max,$min);
The sql giving me wrong result when i pass the limits I do not know why. Is it wrong to specify the max and min of sql query by this way?? Should I correct something in this code?

Answer 1

The LIMIT clause, as explained in the docs, takes arguments offset and count. So if you want to get, for example, results from 201 to 250, you would use LIMIT 200, 50. Start by renaming your variables $min and $max to $offset and $count, and from there everything will fall into place.
Pseudocode:
offset = (requestedPageNumber - 1) * rowsPerPage;
count = rowsPerPage;
PHP Code:
(assuming page number is 0-based)
$rowsPerPage = 50;
$page = empty($_GET['p_n']) ? 0 : $_GET['p_n'];
$offset = $rowsPerPage * (int) $page;
$news = all_news($offset, $rowsPerPage);

Answer 2

If you've got problems handling pagination properly, I suggest you use some code that is working, for example a pagination class that takes three parameters:
The current page.
The total count.
The number of items per page.
And then that class will generate the LIMIT clause for you. Example:
$pageNumber = 1;
$totalCount = 17;
$perPage = 5;
$pagination = new LimitPagination($pageNumber, $totalCount, $perPage);
echo $pagination, "\n";
This would output
LIMIT 0, 5
because you'er on the first page. Such a class then could also filter out those problems you have here, for example setting to a negative page - just automatically. And also it can provide a lot of extra data, like the next and previous page, the current page, the number of total pages, if it is the first or the last page and what not:
$pagination->setPage(-2);
echo $pagination, "\n";
echo "Current: ", $pagination->getPage(),
"; Total: ", $pagination->getTotalPages(),
"; Previous: ", $pagination->getPreviousPage(),
"; Next: ", $pagination->getNextPage(),
"\n";
Output:
LIMIT 0, 5
Current: 1; Total: 4; Previous: 1; Next: 2
This is then easy to integrate with different code, including yours:
$pagination = new LimitPagination($_GET['p_n'], $totalCount, 50);
$limit = sprintf("%d, %d", $pagination->getOffset(), $pagination->getCount());
This should easily do it. Class is here as Gist: https://gist.github.com/4469154

Answer 3

You need to set start (this you can achieve by using current page and per page property) and the second information is how many results you want (again per page property).
LIMIT . ($p_n - 1) * $news_per_page .', ' . $news_per_page
So, in your script it will be:
if(!empty($_GET['p_n']))
{
// You need to protect your variables for SQL injection. For numbers (int) or integer() function it is enough.
$p_n = (int) $_GET['p_n'];
$max = (int) $news_per_page;
$min = (int) ($p_n - 1) * $news_per_page;
}
else
{
$p_n = 1;
$max = 50;
$min = 0;
}

Answer 4

The correct code is:
$news_per_page = 50;
if(!empty($_GET['p_n']))
{
$p_n = intval($_GET['p_n']);
$min = ($p_n-1) * $news_per_page;
$max = $news_per_page;
}
else
{
$p_n = 1;
$max = 50;
$min = 0;
}
$news = all_news($max,$min);
While $_GET['p_n'] is page_number you dont' need to make any multiplies