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Your job is working with documents: creating, modifying, and deleting them. It is a very important task, so you must also keep a changelog of performed operations. Updating it manually every time is very tedious, so you decided to automate the job.

You have to do something, so that all the changes done on the documents table are reflected in the documents_changelog table:

On insert copy the new data into the new_data column
On update copy the previous data into the old_data and the new data into the new_data columns
On delete copy the old data into the old_data column
If the operation has no new/old data to work with, the respective column should store NULL
Note: the id column of the documents table is autoincrementing, hence when new records will be inserted there by the test code, the id's will not be provided explicitly - some language features which could be used for solving this kata may not work because of this fact.

Tables
--------------------------------------------
| Table | Column | Type |
|---------------------+-------------+------|
| documents | id | int |
| | data | text |
|---------------------+-------------+------|
| documents_changelog | id | int |
| | document_id | int |
| | old_data | text |
| | new_data | text |
--------------------------------------------