SQL Prevent Multiple Counting - sql

Question

I found some similar SO questions regarding my issue and they fixed it by using sub-queries but I can't seem to apply it on my situation.
Goal
My goal is to count the animals that have been a breeder at least once in their lifespan.
I have 2 tables to keep track of when an animal became a breeder. Here's a simple look of how the tables are structured:
Animals (a)
id name
-------------------
100 Mouse
101 Cow
102 Pig
103 Dog
Breeding History (bh)
id animal_id code date
--------------------------------------------
500 100 B 2016-01-12
501 100 A 2016-01-25
502 101 B 2016-01-28
503 102 B 2016-02-02
504 100 B 2016-02-05
505 100 A 2016-02-08
In this scenario, my current query for counting works fine for both 101 | Cow and 102 | Pig since they only became a breeder (Code: B) once. The count for an animal who never became a breeder is also correct but it's not really a problem here. For an animal that became a breeder more than once in its lifespan e.g. 100 | Mouse it would be counted by the number of times it became a breeder.
Query
SELECT
a.name,
COUNT(CASE WHEN bh.code IN ('B') THEN 1 ELSE NULL END) AS breeder_count
FROM animals a
LEFT OUTER JOIN breeding_history bh
ON a.id = bh.animal_id
GROUP BY a.name
Result
name breeder_count
--------------------------
Mouse 2
Cow 1
Pig 1
Dog 0
The result shows that there are 2 mice that became a breeder when actually it was the same animal and should only be counted once.

Answer 1

You can use the DISTINCT keyword, so as to count a 'B' just once:
SELECT
a.name,
COUNT(DISTINCT CASE WHEN bh.code IN ('B') THEN 1 END) AS breeder_count
FROM animals a
LEFT OUTER JOIN breeding_history bh
ON a.id = bh.animal_id
GROUP BY a.name
As a side note, ELSE NULL is redundant and has been removed from the CASE expression.
Demo here