Conditional UPDATE statement with JOIN

Question

I'm trying to update a column Plan_Status based on the value of another table main, column Days_At_Step.
The following statements work, however, how would I structure this as a conditional statement?
Or, is there another strategy that I could use?

I feel like there's a better way than using three identical statements with different WHERE conditions.

  UPDATE controlState 
  SET Plan_Status=1 
  FROM controlState JOIN main ON main.Cscc = controlState.Cscc 
  WHERE Days_At_Step < 2;

  UPDATE controlState 
  SET Plan_Status=2 
  FROM controlState JOIN main ON main.Cscc = controlState.Cscc 
  WHERE (Days_At_Step >= 2 OR Days_At_Step < 4);

  UPDATE controlState 
  SET Plan_Status=3 
  FROM controlState JOIN main ON main.Cscc = controlState.Cscc 
  WHERE Days_At_Step > 4;

Answer 1

The three where clauses cover all the possible values Days_At_Step could get, so you really don't need a where clause at all. Instead, you can move this logic to a case expression:

UPDATE controlState 
SET    Plan_Status = CASE WHEN Days_At_Step < 2 THEN 1 
                          WHEN (Days_At_Step >= 2 OR Days_At_Step < 4) THEN 2
                          ELSE 3
                     END
FROM   controlState 
JOIN   main ON main.Cscc = controlState.Cscc;