如何使用Ones数据搜索两个Mysql表

I have tables like this

mainTable

Id | name | country
1  | John | 5
2  | Bill | 7

categoriesTable

other_table_id | category
1              | 6
1              | 12

My question is how can I say

SELECT id FROM mainTable 
WHERE country=5 
AND WHERE categoriesTable order_table_id=[**THE ID I JUST GOT FROM THE FIRST TABLE**] && category=6 || category=12

Then returns the number of records that match so in this case 1

Thanks!

Doesn't anyone learn how to write JOINs when they learn SQL?

SELECT m.id
FROM mainTable AS m
JOIN categoriesTable AS c ON c.other_table_id = m.id
WHERE c.category IN (6, 12)
AND m.country = 5