Can I provide a default for a left outer join? Suppose I have tables a (with column a1) and b (with columns b1 and b2) and I perform a left outer join

SELECT *
FROM a LEFT OUTER JOIN b
ON a.a1 = b.b1


Then b1 and b2 will be NULL where a value of a1 has no matching value of b1.

Can I provide a default value for b2, instead of NULL?  Note that COALESCE won't work here, because I don't want the default value to override potential NULLs in b2 where there is a value of b1 matching a1.

That is, with a and b as

CREATE TABLE a (a1)
  AS VALUES (1),
            (2),
            (3) ;

CREATE TABLE b (b1,b2)
  AS VALUES (1, 10),
            (3, null) ;


a1     b1 | b2
---    --------
 1      1 | 10
 2      3 | NULL
 3


and a default for b2 of, say, 100, I want to get the result

a1 | b1   | b2
---------------
1  |  1   | 10
2  | NULL | 100
3  |  3   | NULL


In this simple case I could do it "by hand" by looking at whether b1 is NULL in the output.  Is that the best option in general, or is there a more standard and neater way?
SELECT a.a1,b.b1,  
    CASE WHEN b.b1 is NULL THEN 5 ELSE b.b2 END AS b2  
FROM a LEFT OUTER JOIN b  
ON a.a1 = b.b1