订购并更改数据库的一列

Question

I have one col in my database named position used for ordering. However, when some records is deleted, the sequence get messed up. I want to reorder this col when the table is changed(maybe use trigger).

position(old)    ->  position(new)
       1                    1
       3                    2
       7                    3
       8                    4

like this. I think there will not exist equal number even in position(old), because I have already attach some function in PHP to reorder the column when updates occurs. However, when a record is deleted because of the deletion of its parent , function will not be called.
Thanks for help!

Answer 1

If you are using the column just for ordering, you do not need to update column on deletion, because the order will still be correct. And you will save some resources.

But if you really need to update by sequence, look at this answer:

updating columns with a sequence number mysql

I believe (as scrowler wrote) the better way in such case is to update rows from the application, after application deletes the parent record.

Answer 2

If you decide to update it in the application then...

If position = n is deleted, you logic should be set position = position - 1 where position > n

Please note that this will work only if you delete one record at a time from your application and before assuming that before the delete is triggered the data is already in sequence