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?

I find COALESCE to be very useful in that case. It will return the first non NULL value from a list:

SELECT
 a.a1,
 b.b1,
 COALESCE (b.b2, 100) AS b2
FROM a
LEFT OUTER JOIN b
  ON (a.a1 = b.b1);

After @ypercube correctly identified an issue where my answer did not match the question. here is his correction:

SELECT 
  a.a1, 
  b.b1, 
  COALESCE(b.b2, d.b2) AS b2   
FROM a LEFT JOIN b ON a.a1 = b.b1 
LEFT JOIN (SELECT 100 AS b2) AS d ON b.b1 IS NULL;

StackExchange Database Administrators Q#56840, answer score: 22