MySQL Crosstab aggregation with 2 conditions - mysql

Question

I have a query which creates a crosstab. The results are a count of the txn_id for branda, and the count of txn_id for brandb.
The txn_id is NOT UNIQUE. This is an example of the transactions table.:
txn_id | nationality_id | sku | sales | units
1 | 1 | 1 | 20 | 2
1 | 1 | 2 | 15 | 1
2 | 4 | 1 | 20 | 2
3 | 2 | 1 | 10 | 1
4 | 3 | 2 | 15 | 1
5 | 4 | 1 | 10 | 1
There are 2 other tables (products) - (sku, brand, product name), and (nationalities) - (nationality_id, nationality).
I would like to add a third column which gets me the count of txn_id where BOTH brands are purchased
The output should be
nationality | branda | brandb | combined
1 | 1 | 1 | 1
2 | 1 | 0 | 0
3 | 0 | 1 | 0
4 | 2 | 0 | 0
Current query.
SELECT
nationalities.nationality,
COUNT((CASE brand WHEN 'branda' THEN txn_id ELSE NULL END)) AS branda,
COUNT((CASE brand WHEN 'brandb' THEN txn_id ELSE NULL END)) AS brandb
<I want my 3rd column here>
FROM
transaction_data
INNER JOIN
products USING (sku)
INNER JOIN
nationalities USING (nationality_id)
GROUP BY nationality
ORDER BY branda DESC
LIMIT 20;
I have tried using:
COUNT((CASE brand WHEN 'brandb' OR 'brandb' THEN txn_id ELSE NULL END)) AS combined - however this obviously returns too many (returns branda or brandb regardless of whether they were purchased together). I know I can't use AND, because obviously no single cell is going to be both branda AND brandb.
I have also tried using:
COUNT((CASE brand WHEN IN('branda', 'brandb') THEN txn_id ELSE NULL END)) AS combined - However this isn't valid syntax.
I feel that I should be using a HAVING clause, but I'm not sure how this would work in the column list.

Answer 1

I think you are going to need two levels of aggregation:
SELECT n.nationality,
sum(branda), sum(brandb), sum(branda * brandb)
FROM (SELECT t.txn_id, n.nationality,
MAX(CASE brand WHEN 'branda' THEN 1 ELSE 0 END) AS branda,
MAX(CASE brand WHEN 'brandb' THEN 1 ELSE 0 END) AS brandb
FROM transaction_data t INNER JOIN
products p
USING (sku) INNER JOIN
nationalities n
USING (nationality_id)
GROUP BY t.txn_id, n.nationality
) tn
GROUP BY n.nationality
ORDER BY max(txn_id) DESC
LIMIT 20;