如何改善分页的SQL查询？

Question

I'm working with the database and servlets, there was such a problem. I need to receive data from the database of 6 pieces per page, for this I made such a request

SELECT *, COUNT(*) AS 'count' 
FROM product
INNER JOIN product_category
  on product.product_category_id = product_category.id 
INNER JOIN  company_manufacturer_product 
  on product.company_manufacturer_product_id =
     company_manufacturer_product.id
GROUP BY 1 LIMIT 6 OFFSET 0;

where 6 is the maximum number of items per page and 0 is the page number multiplied by the maximum quantity of goods. But with such an implementation on the second page I have duplicate products how can i improve it?

The part of the code where I form the request:

StringBuilder startResponse = new StringBuilder("SELECT *, COUNT(*) AS 'count' FROM product " +
                "INNER JOIN product_category on product.product_category_id = product_category.id " +
                "INNER JOIN company_manufacturer_product on product.company_manufacturer_product_id=company_manufacturer_product.id");
if (nonNull(form.getProductMax()) && nonNull(form.getPage())) {
            startResponse.append(" LIMIT ").append(form.getProductMax()).append(" OFFSET ").append(form.getPage() * form.getProductMax());
        }

My database respone without LIMIT and OFFSET:

My database respone when I use the query that described above, this request is sent to the database when I turn to the first page with the goods:

When I turn to the second page with goods, I send such a request to the database

SELECT * , COUNT(*) AS 'count' 
FROM product 
INNER JOIN product_category
  on product.product_category_id = product_category.id
INNER JOIN company_manufacturer_product 
  on product.company_manufacturer_product_id = 
     company_manufacturer_product.id
GROUP BY 1 LIMIT 6 OFFSET 6;

and i have response like that:

I can not understand what the problem is. I have to use requests through COUNT! How prove it?

Answer 1

Not against the solution of this question, according to the above method, adding order by to original sql can solve the problem. But I think I have a better practice for pagination: using parameters like has_more , last_product_id and limit_num to connect clients with server.

has_more indicates more data in server whether or not left; last_product_id indicates the id of last response data; limit_num indicates the number of per page.

So, client can using has_more to determine sending a request or not, if it is, client sends a request with last_product_id and limit_num to server; and for server, the sql can be this:

select * from table where id < $last_product_id order by id desc
limit $limit_num + 1; =>$datas

And, count($datas) and $limit_num to calculate the value of has_more and last_product_id :

$has_more = 0;
$data_num = count($datas);
if ($data_num > $page_limit) {
    $has_more = 1;
    array_pop($datas);
    $data_num--;
}

$last_product_id = end($datas)['id'] ?? 0; 

Answer 2

SELECT *，COUNT（product.id）作为'count'，来自产品INNER JOIN在product.product_category_id上的product_category = product_category.id INNER JOIN company_manufacturer_product在product.company_manufacturer_product_id = company_manufacturer_product.id上按product.id分组按product.id进行订购SET LIMIT 6 OFF 0;