Use max of column and if null use min - sql

Question

I have a table with 10 milestones in the column milestone. The column milestone_achieved has either the value OK or NULL.
The name column has just names, whenever someone new enters, all the milestones are entered in the database with NULL.
Here is what a typical table looks like:
+------+-----------+--------------------+
| name | milestone | milestone_achieved |
+------+-----------+--------------------+
| John | 1 | OK |
| John | 2 | OK |
| John | 3 | NULL |
| John | 4 | NULL |
| John | 5 | NULL |
| John | 6 | NULL |
| Mary | 1 | OK |
| Mary | 2 | OK |
| Mary | 3 | OK |
| Mary | 4 | OK |
| Mary | 5 | OK |
| Mary | 6 | OK |
| Tim | 1 | NULL |
| Tim | 2 | NULL |
| Tim | 3 | NULL |
| Tim | 4 | NULL |
| Tim | 5 | NULL |
| Tim | 6 | NULL |
+------+-----------+--------------------+
Now I want the SQL query to return:
+------+-----------+--------------------+
| name | milestone | milestone_achieved |
+------+-----------+--------------------+
| John | 2 | OK |
| Mary | 6 | OK |
| Tim | 1 | NULL |
+------+-----------+--------------------+
My query right now looks like this:
SELECT name, MAX(milestone) FROM table HAVING milestone_achieved = 'OK' GROUP BY name
UNION ALL
SELECT name, MIN(milestone) FROM table HAVING milestone_achieved IS NULL AND MIN(milestone) = 1 GROUP BY name
This works in 90% of the cases, the problem occurs when e.g. milestone 1 and 2 was completed, but then milestone 1 was "uncompleted" because it didn't fit the specific criteria or whatever (imagine an assembly line where cars are assembled and a screw =milestone 1 isn't tight enough but the paint =milestone 2 is already on it or whatever else you can imagine, I have terrible imagination).
I am now looking fo a way to properly display those 10% cases.

Answer 1

One method is:
SELECT name, MAX(milestone)
FROM table
WHERE milestone_achieved = 'OK'
GROUP BY name
UNION ALL
SELECT name, MIN(milestone)
FROM table
GROUP BY name
HAVING MIN(milestone_achieved) IS NULL;
This follows the structure of your logic. You can do this with one SELECT:
SELECT name,
COALESCE(MAX(CASE WHEN milestone_achieved = 'OK' THEN milestone END),
MIN(milestone)
)
FROM table
GROUP BY name