Question

Suppose we have a simple database containing following data:

        name
        apple
        pear
        banana
        grape

User want to sort those fruits by the name, and we will have, with no surprise

         apple
         banana
         grape
         pear

However, for some reason, user would like to place pear as the 3rd fruit ,that means he would like to have :

         apple
         banana
         pear
         grape

And, importantly, the user want to preserve this order when he want to sort the fruits by name thereafter.

How should we tackle this problem? Of top of my head, we could add a field user_sort_id, which will be updated when user sort and manipulate the sort result and we will use that filed as the sort key.

                       init value       -> sort by name         ->place pear as the seconds
        name           user_sort_id 
        apple            0                     0                          0
        pear             1                     3                          2  
        banana           2                     1                          1  
        grape            3                     2                          3 

This approach should work in theory. However, in practice, I can not think of an elegant and fast SQL statement that could accomplish this. Any ideas or alternatives?

Answer 1

If you want each user to have independent sort orders, you need another table.

CREATE TABLE user_sort_order (
    name VARCHAR(?) NOT NULL REFERENCES your-other-table (name),
    user_id INTEGER NOT NULL REFERENCES users (user_id),
    sort_order INTEGER NOT NULL                -- Could be float or decimal
);

Then ordering is easy.

SELECT name 
FROM user_sort_order
WHERE user_id = ?
ORDER BY sort_order

There’s no magic bullet for updating.

• Delete all the user’s rows, and insert rows with the new order. (Brute force always works.)
• Update every row with the new order. (Could be a lot of UPDATE statements.)
• Track the changes in your app, and update only the changed rows and the rows that have to be “bumped” by the changes. (Parsimonious, but error-prone.)
• Don’t let users impose their own sort order. (Usually not as bad an idea as it sounds.)