How does data migration work in relational databases?

Data migration in relational databases involves transferring data between database systems while maintaining integrity and consistency. This process typically includes three main phases: planning and schema alignment, data transformation and transfer, and validation. The goal is to move data efficiently with minimal downtime, ensuring the target database accurately reflects the source. For example, migrating from MySQL to PostgreSQL requires adjustments for differences in data types, indexing, and SQL dialect.

The first step involves analyzing the source and target schemas to identify structural mismatches. Developers often modify table structures (e.g., converting MySQL’s DATETIME to PostgreSQL’s TIMESTAMP) or adjust constraints (e.g., redefining auto-increment keys using sequences in PostgreSQL). Tools like schema comparison utilities or ORM-generated scripts help automate parts of this process. Data extraction is usually done via SQL exports or dedicated ETL (Extract, Transform, Load) tools. During transformation, data might be cleansed (e.g., removing duplicates), reformatted (e.g., date formats), or restructured (e.g., splitting columns). For large datasets, intermediate storage like CSV files or temporary tables is often used to stage data before loading into the target.

The final phase focuses on validation and testing. Checksums or row-count comparisons verify completeness, while spot-checking sample data ensures accuracy. Foreign key relationships and indexes are re-established post-migration to avoid conflicts during data insertion. Tools like AWS Database Migration Service or open-source options like pgLoader handle incremental transfers and retries for failed operations. For instance, when migrating a customer orders database, developers might test order history queries in the new system to confirm joins and aggregations work as expected. A rollback plan, such as restoring from backups, is critical in case of unexpected issues. Migrations are often performed in maintenance windows or using replication to minimize downtime, with applications gradually redirected to the new database after validation.