Understanding Slowly Changing Dimension Type 2 - data-warehouse

Question

I am having difficult time understanding how to use slowly changing dimension type 2, in my scenario.
I have gone through different tutorial websites but they don't fit.
I have an employee dimension table containing:
+-----+---------------+------------+------------+
| id | employee | designation| Location |
+-----+---------------+------------+------------+
| 1 | Ola | CEO | Newyork |
| 2 | Ahmed | DEVELOPER | California |
| 3 | Ola | Manager | California |
+----------+----------+------------+------------+
I have a Account Fact table
+-------+----------+
|emp_id | Amount |
+-------+-----------
| 1 | 2000000 |
| 2 | 300000 |
+----------+-------+
Now we see that the dimension has changed, and thus a new ID to same Ola employee has been given.
How would we manage in the fact table?
The new ID of Ola will not be found in Fact Table.
so if we add a new row in fact, with new ID of Ola, how would we link that they are same employee, when they are identified differently, 'primary key'.
How would we distinguish this employee is not a new employee and actually location / designation has been changed.

Answer 1

I am sure there are many ways of doing it, here's one way - Have an "employee_Key" in your dimension Table which is unique for an employee. So your dimension table will look like this -
id | emp_key | employee | designation| Location |Valid From| Valid To |
-----|---------|------------|------------|------------|----------|----------|
1 | EMP1 | Ola | CEO | Newyork |1/1/1900 |1/1/2016 |
2 | EMP2 | Ahmed | DEVELOPER | California |1/1/1900 |NULL |
3 | EMP1 | Ola | Manager | California |1/2/2016 |NULL |
You can continue loading your fact table with the "New" ID for the employee. In this case you will have 2 different sets of Keys for that employee.
+-------+----------+
|emp_id | Amount |
| 1 | 2000000 |
| 2 | 300000 |
| 3 | 100000 |
+----------+-------+
If you want to rollup (say Sum of amounts) for an employee from the beginning, you would join the fact and dimension using the ID key and group by emp_key.
So,
select emp_key, sum(amount) from employee dim, account fact where dim.ID = fact.ID group by emp_key.
If you want to find out the amount since he became a manager, you just have to do rollup on the ID field.
select dim.ID, sum(amount) from employee dim, account fact where dim.ID = fact.ID group by dim.ID.
or this way -
select fact.ID, sum(amount) from account fact group by fact.ID.