inner join on pokemon.type = trainer.type에서 type이 electric인 데이터가 pokemon 데이터가 n개, trainer 데이터가 m개인 경우 어떻게 될까?
->
n*m 수만큼의 결과가 반환된다.(type이 electric인 모든 순서쌍 조합들이 만들어진다.)

실험

1. 데이터 구성

pokemon

create table pokemon (
    id INT auto_increment primary key,
    name varchar(20),
    type varchar(20)
);

trainer

create table trainer (
	id BIGINT primary key,
    name varchar(20),
    type varchar(20)
);

pokemon data

id	name	type
1	pikachu	electric
3	pikachu	electric
4	coil	electric

trainer data

id	name	type
1	ash	electric
2	martin	electric

2. 쿼리 실행

select * from trainer inner join pokemon
on trainer.trainer_type=pokemon.type

3. 결과

id	name	type	id	name	type
1	ash	electric	1	pikachu	electric
2	martin	electric	1	pikachu	electric
1	ash	electric	3	pikachu	electric
2	martin	electric	3	pikachu	electric
1	ash	electric	4	coil	electric
2	martin	electric	4	coil	electric