ITransaction trans = this.GetSession().BeginTransaction();
IDbCommand command = this.GetSession().Connection.CreateCommand();
command.CommandText = sql;
command.ExecuteNonQuery();
trans.Commit();

[InvalidOperationException: ExecuteNonQuery requires the command to have a transaction when the connection assigned to the command is in a pending local transaction.  The Transaction property of the command has not been initialized.]
   System.Data.SqlClient.SqlCommand.ValidateCommand(String method, Boolean async) +873221
   System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(DbAsyncResult result, String methodName, Boolean sendToPipe) +72
   System.Data.SqlClient.SqlCommand.ExecuteNonQuery() +135
   ...

SqlConnection does not support parallel transactions.

ITransaction trans = this.GetSession().BeginTransaction();
IQuery query = this.GetSession().CreateSQLQuery(sql, returnAlias, returnClass);
query.List();
this.GetSession().SaveOrUpdate(obj1);
this.GetSession().SaveOrUpdate(obj2);
trans.Commit();

SQL insert, update or delete failed (expected affected row count: 1, actual affected row count: 0). Possible causes: the row was modified or deleted by another user, or a trigger is reporting misleading row count.

ITransaction trans = this.GetSession().BeginTransaction();
IDbCommand command = this.GetSession().Connection.CreateCommand();
command.CommandText = sql;
command.Transaction = trans; // <== add this line
command.ExecuteNonQuery();
trans.Commit();

trans.Enlist(command);