What type of transaction isolation is used by default in the database management system?

By default, most database management systems (DBMS), including SQL Server, use the Read Committed transaction isolation level.

This level guarantees that a transaction only sees data that has been committed by other transactions, preventing dirty reads. However, non-repeatable reads and phantom reads can still occur.

Example in C# using the ReadCommitted isolation level:

using (var transaction = connection.BeginTransaction(System.Data.IsolationLevel.ReadCommitted))
{
    // database operations
    transaction.Commit();
}