What is database migration, and why is it important to manage schema changes through versioned migration files rather than manual changes?

A migration is a versioned, incremental script describing a specific schema change (adding a column, creating a table) that can be applied and, ideally, reversed in a controlled, repeatable, and auditable way -- managing schema changes through migration files (checked into version control alongside application code) rather than manual database edits ensures every environment's schema stays in a known, consistent, and reproducible state, and lets the team track exactly when and why each change was made.

// Example migration file (using a tool like Knex or Sequelize)
exports.up = async (knex) => {
  await knex.schema.table('users', (table) => {
    table.string('phone_number').nullable();
  });
};
exports.down = async (knex) => {
  await knex.schema.table('users', (table) => table.dropColumn('phone_number'));
};