Welling Guzman

Pagination with MySQL

Having a large dataset and only needing to fetch a specific number of rows, it is the reason LIMIT clause exists. It allows to restrict the number of rows in a result returned by a SQL query statement.

Pagination refers to the process of dividing a large dataset into smaller parts.

The ability to send data to the user faster by fetching a whole dataset by small pieces at a time is one of the benefits of using pagination.

How it works

Pagination works by defining the maximum number of rows in the results per request and what page is being requested.

The table below represents the items on a table named users, that is going to be used an example.

+----+----------+
| id | Name     |
+----+----------+
| 1  | John     |
| 2  | Jane     |
| 3  | Peter    |
| 4  | Joseph   |
| 5  | Mary     |
| 6  | Jack     |
| 7  | Ann      |
| 8  | Bill     |
| 9  | Sam      |
| 10 | Rose     |
| 11 | Juan     |
+----+----------+

For this example the maximum number of rows will be 2, which means on every request we are going to get at most 2 rows.

The table has 11 rows, and we are limiting the result by 2 rows per request, resulting in a 6 pages of 2 items. The number of pages are determined by dividing the number of rows (11) by the number of rows per page (2), and making sure the result is rounded to the next integer number.

Total pages = CEIL(Total number of rows / Limit number of rows)

MySQL doesn't have a PAGE clause, but it has a OFFSET clause, which allow to move the position from where to start counting up to the LIMIT number.

The value of OFFSET is done by multiplying the LIMIT clause value by the page number your are looking for minus 1.

OFFSET = LIMIT * (PAGE - 1)

In the table above there is 11 users and to get the first 2 users we use the following query:

PAGE = 1
LIMIT = 2
OFFSET = (PAGE-1) * LIMIT
OFFSET = (1-1) * 2
OFFSET = 0 * 2
OFFSET = 0

The offset initial value is 0, and not 1, that's why we subtract 1 from the page number.

SELECT `id`, `name`
FROM `users`
LIMIT 2
OFFSET 0

The previous query will produce the following result which represents the page 1 of the pagination:

+----+----------+
| id | Name     |
+----+----------+
| 1  | John     |
| 2  | Jane     |
+----+----------+

MySQL has a different way to use offset, without using the OFFSET clause.

SELECT `id`, `name`
FROM `users`
LIMIT 0,2

The first parameter is the offset and the second parameter is the rows count.

To get the second page, or in other word the next two rows, we must calculate again the OFFSET or increase by one the previous value.

PAGE = 2
LIMIT = 2
OFFSET = (PAGE-1) * LIMIT
OFFSET = (2-1) * 2
OFFSET = 1 * 2
OFFSET = 2

SELECT `id`, `name`
FROM `users`
LIMIT 2
OFFSET 2

Below can be seen the result of the previous query:

+----+----------+
| id | Name     |
+----+----------+
| 3  | Peter    |
| 4  | Joseph   |
+----+----------+

The query translate to skip the first 2 items and get the next 2 rows.

So getting the third page we use the following OFFSET of 4, to skip the first 4 items.

PAGE = 3
LIMIT = 2
OFFSET = (PAGE-1) * LIMIT
OFFSET = (3-1) * 2
OFFSET = 2 * 2
OFFSET = 4

SELECT `id`, `name`
FROM `users`
LIMIT 2 OFFSET 4

+----+----------+
| id | Name     |
+----+----------+
| 5  | Mary     |
| 6  | Jack     |
+----+----------+

OFFSET and ORDER BY

Using OFFSET and ORDER BY together could make the pagination non-functional returning in rows random orders, and unexpected rows on each page.

If multiple rows have identical values in the ORDER BY columns, the server is free to return those rows in any order, and may do so differently depending on the overall execution plan. In other words, the sort order of those rows is nondeterministic with respect to the nonordered columns. MySQL documentation

The most common situation is that if you are sorting by a column that doesn't have an index, MySQL Server cannot determine a proper order of the rows.

One way to solve this is by adding an index to the column or columns. Although this may not be as optimal if you don't want or need to add indexes to multiple columns only for this purpose.

If it is important to ensure the same row order with and without LIMIT, include additional columns in the ORDER BY clause to make the order deterministic. MySQL documentation

What this means is there's another way to solve this is by adding to the ORDER BY clause an unique column, for example a primary key column.

SELECT `id`, `name`
FROM `users`
LIMIT 2
OFFSET 2
ORDER BY `name`, `id`

Instead of:

SELECT `id`, `name`
FROM `users`
LIMIT 2
OFFSET 2
ORDER BY `name`

This way you can make sure that MySQL sorts the rows by an unique column before finding the LIMIT number of rows.

 
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