Database Migration — Explained with Examples

A database migration is a version-controlled change to a database schema, allowing teams to evolve the database incrementally alongside application code.

Database Migration (also called schema migration or evolution) is a technique for managing incremental, reversible changes to a database schema. Tools like Flyway (Java), Alembic (Python), EF Migrations (.NET), and Prisma Migrate (TypeScript) automate this process.

Why Migration Matters

Without migrations, schema changes are applied manually — leading to drift between environments, missing columns in production, and no audit trail. Migrations version-control the schema alongside application code, so every environment stays in sync and changes are reproducible.

Real-World Analogy

Renovating a house while living in it. You don’t demolish the entire house and rebuild (drop and recreate the database). Instead, you change one room at a time (add a column, rename a table), document each change, and ensure the house remains livable during construction.

Example: Alembic Migration

"""Migration: add user roles table

Revision ID: 0002
Create Date: 2026-06-15
"""
from alembic import op
import sqlalchemy as sa

# Upgrade — apply the change
def upgrade():
    op.create_table(
        'roles',
        sa.Column('id', sa.Integer(), primary_key=True),
        sa.Column('name', sa.String(50), nullable=False),
        sa.Column('user_id', sa.Integer(), sa.ForeignKey('users.id')),
    )

# Downgrade — revert the change
def downgrade():
    op.drop_table('roles')

# Usage:
# alembic upgrade head     → applies all pending migrations
# alembic downgrade -1     → reverts the last migration

Related Terms

CRUD, ORM, Replication