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+---
+name: database-migration
+description: Execute database migrations across ORMs and platforms with zero-downtime strategies, data transformation, and rollback procedures. Use when migrating databases, changing schemas, performing data transformations, or implementing zero-downtime deployment strategies.
+---
+
+# Database Migration
+
+Master database schema and data migrations across ORMs (Sequelize, TypeORM, Prisma), including rollback strategies and zero-downtime deployments.
+
+## When to Use This Skill
+
+- Migrating between different ORMs
+- Performing schema transformations
+- Moving data between databases
+- Implementing rollback procedures
+- Zero-downtime deployments
+- Database version upgrades
+- Data model refactoring
+
+## ORM Migrations
+
+### Sequelize Migrations
+```javascript
+// migrations/20231201-create-users.js
+module.exports = {
+  up: async (queryInterface, Sequelize) => {
+    await queryInterface.createTable('users', {
+      id: {
+        type: Sequelize.INTEGER,
+        primaryKey: true,
+        autoIncrement: true
+      },
+      email: {
+        type: Sequelize.STRING,
+        unique: true,
+        allowNull: false
+      },
+      createdAt: Sequelize.DATE,
+      updatedAt: Sequelize.DATE
+    });
+  },
+
+  down: async (queryInterface, Sequelize) => {
+    await queryInterface.dropTable('users');
+  }
+};
+
+// Run: npx sequelize-cli db:migrate
+// Rollback: npx sequelize-cli db:migrate:undo
+```
+
+### TypeORM Migrations
+```typescript
+// migrations/1701234567-CreateUsers.ts
+import { MigrationInterface, QueryRunner, Table } from 'typeorm';
+
+export class CreateUsers1701234567 implements MigrationInterface {
+  public async up(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.createTable(
+      new Table({
+        name: 'users',
+        columns: [
+          {
+            name: 'id',
+            type: 'int',
+            isPrimary: true,
+            isGenerated: true,
+            generationStrategy: 'increment'
+          },
+          {
+            name: 'email',
+            type: 'varchar',
+            isUnique: true
+          },
+          {
+            name: 'created_at',
+            type: 'timestamp',
+            default: 'CURRENT_TIMESTAMP'
+          }
+        ]
+      })
+    );
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.dropTable('users');
+  }
+}
+
+// Run: npm run typeorm migration:run
+// Rollback: npm run typeorm migration:revert
+```
+
+### Prisma Migrations
+```prisma
+// schema.prisma
+model User {
+  id        Int      @id @default(autoincrement())
+  email     String   @unique
+  createdAt DateTime @default(now())
+}
+
+// Generate migration: npx prisma migrate dev --name create_users
+// Apply: npx prisma migrate deploy
+```
+
+## Schema Transformations
+
+### Adding Columns with Defaults
+```javascript
+// Safe migration: add column with default
+module.exports = {
+  up: async (queryInterface, Sequelize) => {
+    await queryInterface.addColumn('users', 'status', {
+      type: Sequelize.STRING,
+      defaultValue: 'active',
+      allowNull: false
+    });
+  },
+
+  down: async (queryInterface) => {
+    await queryInterface.removeColumn('users', 'status');
+  }
+};
+```
+
+### Renaming Columns (Zero Downtime)
+```javascript
+// Step 1: Add new column
+module.exports = {
+  up: async (queryInterface, Sequelize) => {
+    await queryInterface.addColumn('users', 'full_name', {
+      type: Sequelize.STRING
+    });
+
+    // Copy data from old column
+    await queryInterface.sequelize.query(
+      'UPDATE users SET full_name = name'
+    );
+  },
+
+  down: async (queryInterface) => {
+    await queryInterface.removeColumn('users', 'full_name');
+  }
+};
+
+// Step 2: Update application to use new column
+
+// Step 3: Remove old column
+module.exports = {
+  up: async (queryInterface) => {
+    await queryInterface.removeColumn('users', 'name');
+  },
+
+  down: async (queryInterface, Sequelize) => {
+    await queryInterface.addColumn('users', 'name', {
+      type: Sequelize.STRING
+    });
+  }
+};
+```
+
+### Changing Column Types
+```javascript
+module.exports = {
+  up: async (queryInterface, Sequelize) => {
+    // For large tables, use multi-step approach
+
+    // 1. Add new column
+    await queryInterface.addColumn('users', 'age_new', {
+      type: Sequelize.INTEGER
+    });
+
+    // 2. Copy and transform data
+    await queryInterface.sequelize.query(`
+      UPDATE users
+      SET age_new = CAST(age AS INTEGER)
+      WHERE age IS NOT NULL
+    `);
+
+    // 3. Drop old column
+    await queryInterface.removeColumn('users', 'age');
+
+    // 4. Rename new column
+    await queryInterface.renameColumn('users', 'age_new', 'age');
+  },
+
+  down: async (queryInterface, Sequelize) => {
+    await queryInterface.changeColumn('users', 'age', {
+      type: Sequelize.STRING
+    });
+  }
+};
+```
+
+## Data Transformations
+
+### Complex Data Migration
+```javascript
+module.exports = {
+  up: async (queryInterface, Sequelize) => {
+    // Get all records
+    const [users] = await queryInterface.sequelize.query(
+      'SELECT id, address_string FROM users'
+    );
+
+    // Transform each record
+    for (const user of users) {
+      const addressParts = user.address_string.split(',');
+
+      await queryInterface.sequelize.query(
+        `UPDATE users
+         SET street = :street,
+             city = :city,
+             state = :state
+         WHERE id = :id`,
+        {
+          replacements: {
+            id: user.id,
+            street: addressParts[0]?.trim(),
+            city: addressParts[1]?.trim(),
+            state: addressParts[2]?.trim()
+          }
+        }
+      );
+    }
+
+    // Drop old column
+    await queryInterface.removeColumn('users', 'address_string');
+  },
+
+  down: async (queryInterface, Sequelize) => {
+    // Reconstruct original column
+    await queryInterface.addColumn('users', 'address_string', {
+      type: Sequelize.STRING
+    });
+
+    await queryInterface.sequelize.query(`
+      UPDATE users
+      SET address_string = CONCAT(street, ', ', city, ', ', state)
+    `);
+
+    await queryInterface.removeColumn('users', 'street');
+    await queryInterface.removeColumn('users', 'city');
+    await queryInterface.removeColumn('users', 'state');
+  }
+};
+```
+
+## Rollback Strategies
+
+### Transaction-Based Migrations
+```javascript
+module.exports = {
+  up: async (queryInterface, Sequelize) => {
+    const transaction = await queryInterface.sequelize.transaction();
+
+    try {
+      await queryInterface.addColumn(
+        'users',
+        'verified',
+        { type: Sequelize.BOOLEAN, defaultValue: false },
+        { transaction }
+      );
+
+      await queryInterface.sequelize.query(
+        'UPDATE users SET verified = true WHERE email_verified_at IS NOT NULL',
+        { transaction }
+      );
+
+      await transaction.commit();
+    } catch (error) {
+      await transaction.rollback();
+      throw error;
+    }
+  },
+
+  down: async (queryInterface) => {
+    await queryInterface.removeColumn('users', 'verified');
+  }
+};
+```
+
+### Checkpoint-Based Rollback
+```javascript
+module.exports = {
+  up: async (queryInterface, Sequelize) => {
+    // Create backup table
+    await queryInterface.sequelize.query(
+      'CREATE TABLE users_backup AS SELECT * FROM users'
+    );
+
+    try {
+      // Perform migration
+      await queryInterface.addColumn('users', 'new_field', {
+        type: Sequelize.STRING
+      });
+
+      // Verify migration
+      const [result] = await queryInterface.sequelize.query(
+        "SELECT COUNT(*) as count FROM users WHERE new_field IS NULL"
+      );
+
+      if (result[0].count > 0) {
+        throw new Error('Migration verification failed');
+      }
+
+      // Drop backup
+      await queryInterface.dropTable('users_backup');
+    } catch (error) {
+      // Restore from backup
+      await queryInterface.sequelize.query('DROP TABLE users');
+      await queryInterface.sequelize.query(
+        'CREATE TABLE users AS SELECT * FROM users_backup'
+      );
+      await queryInterface.dropTable('users_backup');
+      throw error;
+    }
+  }
+};
+```
+
+## Zero-Downtime Migrations
+
+### Blue-Green Deployment Strategy
+```javascript
+// Phase 1: Make changes backward compatible
+module.exports = {
+  up: async (queryInterface, Sequelize) => {
+    // Add new column (both old and new code can work)
+    await queryInterface.addColumn('users', 'email_new', {
+      type: Sequelize.STRING
+    });
+  }
+};
+
+// Phase 2: Deploy code that writes to both columns
+
+// Phase 3: Backfill data
+module.exports = {
+  up: async (queryInterface) => {
+    await queryInterface.sequelize.query(`
+      UPDATE users
+      SET email_new = email
+      WHERE email_new IS NULL
+    `);
+  }
+};
+
+// Phase 4: Deploy code that reads from new column
+
+// Phase 5: Remove old column
+module.exports = {
+  up: async (queryInterface) => {
+    await queryInterface.removeColumn('users', 'email');
+  }
+};
+```
+
+## Cross-Database Migrations
+
+### PostgreSQL to MySQL
+```javascript
+// Handle differences
+module.exports = {
+  up: async (queryInterface, Sequelize) => {
+    const dialectName = queryInterface.sequelize.getDialect();
+
+    if (dialectName === 'mysql') {
+      await queryInterface.createTable('users', {
+        id: {
+          type: Sequelize.INTEGER,
+          primaryKey: true,
+          autoIncrement: true
+        },
+        data: {
+          type: Sequelize.JSON  // MySQL JSON type
+        }
+      });
+    } else if (dialectName === 'postgres') {
+      await queryInterface.createTable('users', {
+        id: {
+          type: Sequelize.INTEGER,
+          primaryKey: true,
+          autoIncrement: true
+        },
+        data: {
+          type: Sequelize.JSONB  // PostgreSQL JSONB type
+        }
+      });
+    }
+  }
+};
+```
+
+## Resources
+
+- **references/orm-switching.md**: ORM migration guides
+- **references/schema-migration.md**: Schema transformation patterns
+- **references/data-transformation.md**: Data migration scripts
+- **references/rollback-strategies.md**: Rollback procedures
+- **assets/schema-migration-template.sql**: SQL migration templates
+- **assets/data-migration-script.py**: Data migration utilities
+- **scripts/test-migration.sh**: Migration testing script
+
+## Best Practices
+
+1. **Always Provide Rollback**: Every up() needs a down()
+2. **Test Migrations**: Test on staging first
+3. **Use Transactions**: Atomic migrations when possible
+4. **Backup First**: Always backup before migration
+5. **Small Changes**: Break into small, incremental steps
+6. **Monitor**: Watch for errors during deployment
+7. **Document**: Explain why and how
+8. **Idempotent**: Migrations should be rerunnable
+
+## Common Pitfalls
+
+- Not testing rollback procedures
+- Making breaking changes without downtime strategy
+- Forgetting to handle NULL values
+- Not considering index performance
+- Ignoring foreign key constraints
+- Migrating too much data at once