Explain the concept of nested transactions.

Nested transactions involve a hierarchical structure of transactions where one transaction (parent) contains one or more sub-transactions (children). Each sub-transaction can be executed independently but is part of the larger parent transaction. The key concepts are:

1. **Parent and Child Transactions:** A parent transaction can have multiple child transactions. Child transactions are committed or rolled back as a group, but their individual outcomes do not affect the parent transaction until the parent itself is committed.

2. **Isolation:** Each nested transaction is isolated from others, meaning changes made by a child transaction are not visible to other transactions until the parent transaction is committed.

3. **Commit and Rollback:** If a parent transaction commits, all its child transactions are also committed. If a rollback occurs, all changes made by the child transactions are undone, maintaining consistency.

4. **Error Handling:** Nested transactions allow finer control over error handling and recovery. If a child transaction fails, only the child is affected, and the parent can decide whether to continue or abort the entire process.

Nested transactions enhance modularity, fault tolerance, and manageability in complex transaction processing scenarios.