Filling Blank/Null values with other values within a column - sql

Question

Ive got a column that is set up like so:
-----Col_2-----
Value 1
Value 2
-----------------
As you can see there are 2 values within the column, but there are also many spaces as well. What I would like to do is run an Update query that would replace the blanks with the 2 values. What I would like is to update the column to look like this:
-------Col_2---------
Value 1
Value 1
Value 1
Value 2
Value 2
value 2
---------------------
Basically I want to run a query that looks at the first not null value and replaces the following null values with that non null value until it hits the next non null value. Once it hits the next non null value, it then proceeds to replace the following nulls with the new value. Currently, I've got this set up.
Update Table1,
(Select TOP 1 Col_1 AS Z FROM Table1 Where Col_1 Is Not Null)
Set Col_1 = Z
Where Col_1 Is Null;
This replaces every null value with whatever the first non null value is, but doesn't stop once it hits the next non null value. This results in me getting something like this:
---------Col_2-----------
Value 1
Value 1
Value 1
Value 2
Value 1
Value 1
-------------------------
Is there a way to add some sort of "do this until" this statement into access? I know you can run if, then statements in PHP and other languages, but not sure how to do this in access or straight SQL.
UPDATE:
There is a second column that contains account numbers, which correlate to the values in COL_1. The original set up looks like this.
-----Col_1-------Col_2------
12345 Value 1
12345
12345
54321 Value 2
54321
54321
------------------------------
Is there a way to use Col_1 in order to get Col_2 to look like:
--------Col_2---------
Value 1
Value 1
Value 1
Value 2
Value 2
Value 2
----------------------
without having to manually enter Col_1's numbers into the query? By this I mean, I don't want to have to manually enter "12345" or "54321" in the query, but rather have things happen automatically.

Answer 1

Assuming I'm understanding your question correctly, you can use an update statement, joining the table back to itself. I think this is the correct syntax in Access:
update Table1
inner join Table1 t2 on Table1.col1 = t2.col1 and t2.col2 is not null
set Table1.col2 = t2.col2
SQL Fiddle Demo

Answer 2

One way of doing it:
UPDATE SomeData SDOuter
SET Col_2 = (SELECT MAX(SDInner.Col_2) FROM SomeData SDInner WHERE SDInner.Col_1 = SDOuter.Col_1 AND Col_2 IS NOT NULL)
WHERE Col_2 IS NULL;
SQL Fiddle