When working with MySQL, it's common to use pagination to limit the amount of data fetched in a single query, especially when dealing with large datasets. For example, you might limit results using LIMIT and OFFSET to display a specific page of records. Additionally, sorting the data is often necessary to display it in a meaningful order, using ORDER BY on one or more columns.

However, one particular edge case can lead to unexpected behavior and headaches: when the column you're sorting by contains NULL values. In this post, we'll explore why sorting on nullable columns in MySQL can cause issues, especially when combined with pagination, and how you can mitigate these problems.

The problem: NULL values and sort order

In MySQL, NULL represents the absence of a value, and when used in ORDER BY clauses, it can introduce non-intuitive sorting behavior. By default, MySQL considers NULL values to be lower than any non-NULL value when sorting in ascending order. Conversely, when sorting in descending order, NULL values are considered higher than any non-NULL value.

Here's where the trouble starts: MySQL does not guarantee a consistent order for rows with the same values or NULL values. This becomes problematic when:

• You have a dataset with rows that contain NULL in the column you are sorting by.
• You combine sorting with pagination (i.e., using LIMIT and OFFSET to display results page-by-page).

In this scenario, rows containing NULL might appear in unpredictable places across different pages.

Example: a misbehaving pagination query

Let's say you have a table called products with the following data:

id	name	price
1	Widget A	10.00
2	Widget B	NULL
3	Widget C	15.00
4	Widget D	NULL
5	Widget E	20.00

You want to sort the products by the price column in ascending order and paginate the results using LIMIT and OFFSET. Here's your query:

SELECT * FROM products ORDER BY price ASC LIMIT 2 OFFSET 0;

This might return:

id	name	price
2	Widget B	NULL
4	Widget D	NULL

If you now fetch the next page using:

SELECT * FROM products ORDER BY price ASC LIMIT 2 OFFSET 2;

You might expect:

id	name	price
1	Widget A	10.00
3	Widget C	15.00

However, due to the nature of NULL values and how MySQL handles sorting, you might be surprised to find that Widget B and Widget D show up again in this page, possibly shifting records around. This is because rows with NULL values are not sorted in a deterministic manner unless you explicitly define their position in the sort order.

Why this happens

MySQL's default behavior when sorting is to leave records with the same sort value (or NULL) in an undefined order unless a secondary criterion is specified. This means that when you query a dataset with NULL values, MySQL could place those NULL rows in varying positions across different query executions or paginated results.

The query results can change between requests as you paginate through the data, leading to records being duplicated or skipped between pages.

Solutions to avoid pagination surprises

1. Explicitly handle NULL values in the sort order: You can modify your query to handle NULL values more explicitly by using IS NULL or a COALESCE function to replace NULL values with a placeholder value during sorting.

   Here's a safer query that avoids undefined behavior:

   SELECT * FROM products ORDER BY price IS NULL, price ASC LIMIT 2 OFFSET 0;
   

   In this query, price IS NULL will ensure that rows with NULL values are placed at the end of the result set.

2. Provide a secondary sort criterion: Another way to ensure deterministic sorting is to add a secondary sort criterion. For example, sorting by id as a fallback:

   SELECT * FROM products ORDER BY price ASC, id ASC LIMIT 2 OFFSET 0;
   

   This will ensure that rows with the same price (or NULL values) are sorted by id, providing consistency across pages.

3. Use COALESCE to replace NULL values: If you want NULL values to be treated as a specific number, such as 0, you can use the COALESCE function:

   SELECT * FROM products ORDER BY COALESCE(price, 0) ASC LIMIT 2 OFFSET 0;
   

   In this query, all NULL values are treated as 0, avoiding undefined behavior.

Conclusion

When using sorting combined with pagination in MySQL, handling NULL values properly is critical to avoid surprises. Failing to address how NULL values are sorted can result in non-deterministic pagination results, where records may be duplicated or omitted across pages.

By explicitly handling NULL values in your ORDER BY clause and adding secondary sorting criteria, you can ensure consistent and predictable pagination behavior. This small but important detail can save you from bugs, user confusion, and unpredictable data presentation in your applications.