Usage of COALESCE - mysql

Question

I have a query making use of COALESCE() to count the combination of 2 columns:
SELECT method, main_ingredient, COUNT(*) AS cnt FROM `recipes`
GROUP BY COALESCE( method, main_ingredient )
The result is useful. Sample result:
method main_ingredient cnt
================================
1 4 10
2 1 6
3 6 3
4 6 5
5 2 4
6 8 2
However, how can I obtain the results that has COUNT(*) equals to 0 ?
UPDATE with expected output:
method main_ingredient cnt
================================
1 2 0
1 3 0
1 5 0
1 6 0
2 2 0
2 3 0
.
.
.
.
etc
UPDATE added the tbl_methods and tbl_main_ingredients:
Schema of tbl_methods:
id method_name
=================
1 Method 1
2 Method 2
.
.
.
6 Method 6
Schema of tbl_main_ingredients:
id ingredient_name
======================
1 Ingredient 1
2 Ingredient 2
.
.
.
8 Ingredient 8
Both id are the primary key of their table, auto-increment.

Answer 1

First you need to make a CROSS JOIN between tbl_methods and tbl_main_ingredients table in order to obtain the all possible combination of method and ingredient.
Later make a left join between the above cross joined table and your reipes table on matching method and main_ingredient.
Thus you will obtain a result for all possible combination of method and main_ingredient. If any combination exists in recipes table then you will get the corresponding count otherwise you will obtain 0 as count.
SELECT
method_ingredients.method_id,
method_ingredients.ingredients_id,
COUNT(R.method) AS cnt
FROM
(
SELECT
TM.id AS method_id,
TMI.id AS ingredients_id
FROM tbl_methods TM
CROSS JOIN tbl_main_ingredients TMI
) AS method_ingredients
LEFT JOIN `recipes` R ON R.method = method_ingredients.method_id AND R.main_ingredient = method_ingredients.ingredients_id
GROUP BY method_ingredients.method_id, method_ingredients.ingredients_id
ORDER BY method_ingredients.method_id, method_ingredients.ingredients_id;
Or
you can prefer the shorter version of this query:
SELECT
TM.id AS method_id,
TMI.id AS ingredients_id,
COUNT(R.method) AS cnt
FROM tbl_methods TM
CROSS JOIN tbl_main_ingredients TMI
LEFT JOIN `recipes` R ON R.method = TM.id AND R.main_ingredient = TMI.id
GROUP BY TM.id, TMI.id
ORDER BY TM.id, TMI.id;
More:
Some subtleties regarding COUNT:
SELECT COUNT(0); Result: 1
SELECT COUNT(-1); Result: 1
SELECT COUNT(NULL); Result: 0
SELECT COUNT(71); Result: 1
SQL FIDDLE
BTW there's nothing to do with COALESCE in your use case. COALESCE returns the first non-NULL element from the list if there's any otherwise NULL.
Example:
SELECT COALESCE(NULL,NULL,NULL,'abc',NULL,'def'); returns abc
SELECT COALESCE(NULL,NULL,NULL); returns NULL

Answer 2

Could be you need to check if the main_ingredient is null
SELECT method, ifnull(main_ingredient,0), COUNT(*) AS cnt FROM `recipes`
GROUP BY method

Answer 3

Cross join your 2 base tables, then left join on recipes. Then, if you count any of the left joined columns, you will get the desired result:
select m.id, i.id, count(r.method) as cnt
from tbl_methods m
cross join tbl_main_ingredients i
left join recipes r
on r.method = m.id
and r.main_ingredient = i.id
group by m.id, i.id
order by m.id, i.id