一对一关系平整计数

Question

I have two tables, Person and Hobby , with a one to many relationship.

| ID | Person |     | ID | PersonID | Hobby      |   
|----|--------|     |----|----------|------------|
| 1  | Bob    |     | 1  | 1        | Skiing     |
| 2  | Mike   |     | 2  | 2        | Biking     |  
| 3  | Frank  |     | 3  | 3        | Skiing     |
                    | 4  | 1        | Numerology |
                    | 5  | 2        | Witchcraft |
                    | 6  | 3        | Stamps     |

I want to produce a table that includes a single count for every person who likes Skiing or Stamps. Something like this:

| Person | Count |
|--------|-------|
| Bob    | 1     |
| Frank  | 1     |

What would a working sql query look like?

Answer 1

select p.person, count(p.id) as cnt
from person p join hobby h
on h.personid = p.id
where h.hobby = 'Skiing'
group by p.person

You have to join the tables and group by the person id.

Answer 2

This solved it. You need to count distinct

SELECT p.person, count(Distinct p.id) as Count
FROM person p join hobby h
   ON h.personID = p.ID
WHERE h.hobby = 'Skiing' 
   OR h.hobby = 'Stamps'
GROUP BY p.ID