Database Migration Practice: Master Schema Evolution

The Migration Challenge

Most tutorials teach you to build databases from scratch. But real production work is different:

Tutorial: Create a perfect schema Reality: Evolve an existing schema while preserving data

This is one of the hardest and most critical backend skills. And it's almost impossible to practice without a real evolving system.

Why Migrations Are Hard

You Can't Start Over

In production, you have:

  • Live data users depend on
  • Running services that expect certain schemas
  • No downtime tolerance
  • No "just drop and recreate" option

Requirements Change Constantly

Real systems evolve:

  • New features need new fields
  • Relationships change
  • Performance requires denormalization
  • Business logic evolves

You Must Preserve Everything

Migrations must:

  • Transform existing data correctly
  • Maintain referential integrity
  • Preserve all historical data
  • Work across all environments

Backward Compatibility Matters

Often you need:

  • Old API version still working
  • Gradual rollout of changes
  • Rollback capability
  • Zero downtime deployments

Most Platforms Don't Teach This

LeetCode/HackerRank: No persistent data, no migrations

Tutorials: Usually start fresh every time

Bootcamps: Teach schema creation, not evolution

Side Projects: Rarely reach the complexity where migrations matter

YoloCorp: Migration Bootcamp

Every YoloCorp project forces schema evolution.

Episode 1: Initial Schema

You design your database:

CREATE TABLE users (

  id UUID PRIMARY KEY,

  name TEXT NOT NULL,

  email TEXT UNIQUE NOT NULL

);

Episode 5: Requirements Change

New feature requires user roles:

-- Can't just add a column - need to:

-- 1. Add column with default value

-- 2. Migrate existing data based on business rules

-- 3. Remove default after backfill

-- 4. Maybe add constraints



ALTER TABLE users ADD COLUMN role TEXT DEFAULT 'user';

-- Update existing data based on episode-specific logic

UPDATE users SET role = 'admin' WHERE id IN (...);

ALTER TABLE users ALTER COLUMN role SET NOT NULL;

ALTER TABLE users ALTER COLUMN role DROP DEFAULT;

Episode 8: Relationship Changes

Users now have organizations:

-- Must preserve user data while adding relationships

CREATE TABLE organizations (...);

CREATE TABLE user_organizations (...);

-- Migrate existing users into default organizations

-- Update application code to handle new structure

Real Migration Scenarios

Adding Required Fields

Challenge: Can't add NOT NULL column to table with data Solution: Multi-step migration with defaults and backfills

Changing Relationships

Challenge: Data model refactor (1-to-1 becomes 1-to-many) Solution: Create new tables, migrate data, update references, deprecate old

Denormalization for Performance

Challenge: Queries too slow, need to duplicate data Solution: Add computed columns, backfill, maintain consistency

Removing Features

Challenge: Column/table no longer needed but can't just drop Solution: Deprecation strategy, data archival, careful removal

Splitting Tables

Challenge: Single table grew too complex, needs normalization Solution: Extract to new table, migrate data, update foreign keys

Data Type Changes

Challenge: TEXT field needs to become ENUM or INT Solution: Validate existing data, transform, handle invalid cases

Migration Patterns You'll Learn

Forward Migration

-- Up: Apply changes

ALTER TABLE products ADD COLUMN stock_count INTEGER DEFAULT 0;

Rollback Migration

-- Down: Undo changes safely

ALTER TABLE products DROP COLUMN stock_count;

Data Transformation

-- Migrate data format

UPDATE users

SET phone = format_phone(old_phone)

WHERE old_phone IS NOT NULL;

Multi-Step Migrations

-- Step 1: Add nullable column

ALTER TABLE users ADD COLUMN verified BOOLEAN;



-- Step 2: Backfill data (run application logic)

-- (Application sets values based on business rules)



-- Step 3: Make non-nullable

ALTER TABLE users ALTER COLUMN verified SET NOT NULL;

Zero-Downtime Patterns

-- Old and new columns coexist

ALTER TABLE orders ADD COLUMN status_v2 TEXT;

-- Application writes to both, reads from new

-- After deploy, remove old column

Skills You'll Develop

Schema Design for Evolution

  • Designing schemas that can change
  • Anticipating future needs
  • Planning for flexibility
  • Avoiding migration traps

Migration Strategy

  • Breaking changes into safe steps
  • Ordering operations correctly
  • Handling failures gracefully
  • Testing migrations thoroughly

Data Integrity

  • Maintaining referential integrity
  • Preserving data relationships
  • Validating data transformations
  • Handling edge cases

Production Safety

  • Zero-downtime techniques
  • Rollback strategies
  • Backup and restore
  • Risk mitigation

Database-Specific Features

  • Postgres migrations vs MySQL
  • Transaction handling
  • Index management during migrations
  • Lock prevention

YoloCorp Forces Best Practices

Must Preserve Data

Tests verify data continuity across episodes. Drop the table? Tests fail.

Must Handle Edge Cases

Your migration code must handle:

  • NULL values
  • Duplicates
  • Invalid data
  • Missing references

Must Plan Ahead

Episode 10 requirements might make episode 3 decisions painful. Learn to design for change.

Must Debug Production Issues

Migration failed? Container won't start? Tests fail mysteriously? Debug it.

Real Interview Questions

YoloCorp prepares you for questions like:

"How do you add a NOT NULL column to a table with millions of rows?" You'll have done it. You know the multi-step pattern.

"How do you migrate from one data model to another with zero downtime?" You've lived through refactoring episodes. You know the strategies.

"What do you do if a migration fails halfway through?" You've debugged failed migrations. You understand transactions and rollback.

"How do you handle schema versioning?" You've evolved schemas across episodes. You understand version management.

Practice Scenarios

YoloCorp projects might force you to:

Add Computed Fields

Episode 6 requires instant access to data that requires expensive joins. Add denormalized columns and keep them synchronized.

Change Primary Keys

Episode 7 reveals UUID isn't the right choice. Migrate to different ID strategy without losing data.

Split Monolithic Tables

Episode 9's complexity means your Users table is doing too much. Extract concerns without breaking existing code.

Add Constraints

Episode 4 requires unique constraints on data that isn't currently unique. Clean data, then add constraint.

Beyond YoloCorp

These skills apply everywhere:

Postgres, MySQL, MongoDB - Migration concepts are universal

ORMs - Learn to use Prisma, TypeORM, Django migrations, ActiveRecord

Production Databases - Know how to safely evolve real systems

Cloud Platforms - AWS RDS, Google Cloud SQL require same skills

Start Practicing

You can't learn migrations from theory. You need a system that evolves.

Download the CLI and start a project.

By episode 10, you'll have migrated schemas multiple times. You'll understand the pain points, the patterns, and the solutions.

Perfect for: Developers wanting to level up database skills, engineers preparing for senior roles, anyone who needs to understand production database evolution.