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MongoDB is considered flexible in its schema because a document's structure doesn't "freeze" upfront the way it does in relational databases.

### The key reasons

1. **No fixed tables and columns**
In MongoDB you don't need to describe a schema upfront. Documents in the same collection can have different sets of fields - the database won't complain if one document has a `phone` field and another doesn't.
2. **Easy to change the data structure**
Adding a new field doesn't require migrating the whole collection. You just start writing the new field into new documents, while the old ones stay as they are.
3. **Support for nested structures**
The document model lets you store arrays and nested objects right inside the document. That removes the need for complex JOINs and rigid relationships.
4. **Different data versions can coexist**
The application can be updated gradually: some data is already in the new structure, some is still in the old one, and that doesn't break the system.

**The essence of it:** MongoDB gives you freedom, data can evolve alongside the application without constant rigid migrations and schema changes.

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