How are data migration processes carried out in databases?

Data migration in databases is the process of changing the database structure or transferring data between different schema versions. Migrations are typically used to add new tables, modify columns, indexes, etc.

In Python, migrations are often implemented using ORM, such as Django ORM or Alembic for SQLAlchemy. The process includes:

1. Creating a migration file that describes the schema changes.
2. Applying the migration to the database with a command that updates the structure.
3. Rolling back the migration if necessary.

Example with Alembic:

alembic revision -m "add new column"
alembic upgrade head

Inside the migration file, it describes what changes need to be made, for example, adding a column:

op.add_column('user', sa.Column('age', sa.Integer))

Thus, migrations allow managing database changes in a sequential and safe manner.