WHY DIVIDE LEVELS? Data Independence Physical Independence The index and storage method can be changed without modifying queries, provided that the database interface remains the same. This is the goal, not an automatic guarantee. Logical Independence A VIEW can preserve the read interface when tables are modified. Write operations, migrations, and application compatibility are verified separately. What's Possible in Practice • The index was added • split the table into two; reports were not rewritten • We switched the database server without changing the interface • the new field didn't break the backend Independence requires a stable interface and validation of changes.