N×M個表查詢

Question

爲了ilustrate我的問題，考慮這三個表：＃

人：

personid int auto_increment not null, 
firstname varchar(16) not null, 
constraint pk_person primary key (personid) 

寵物：

petid int auto_increment not null, 
petname varchar(16) not null, 
constraint pk_pet primary key (petid) 

所有權：

owner int not null, 
pet int not null, 
constraint fk_owner_ownership foreign key (owner) references Person (personid) on delete cascade, 
constraint fk_pet_ownership foreign key (pet) references Pet (petid) on delete cascade, 
constraint pk_ownership primary key (owner, pet) 

和元組：

insert into person (firstname) values ("andy"); 
insert into person (firstname) values ("barney"); 
insert into person (firstname) values ("carly"); 

insert into pet (petname) values ("dog"); 
insert into pet (petname) values ("cat"); 

insert into ownership (owner, pet) values (1, 1); #andy owns a dog 
insert into ownership (owner, pet) values (2, 2); #barney owns a cat 
insert into ownership (owner, pet) values (3, 1); 
insert into ownership (owner, pet) values (3, 2); #carly owns a dog and a cat 

我想要一個查詢，只返回擁有狗和貓，在這種情況下是carly的所有者。寵物的數量可能超過這兩個。

Answer 1

有幾種方法可以做到這一點，包括使用兩個exists條件。然而，我個人最喜歡的是查詢哪些業主有貓或狗，並計算他們擁有的不同寵物數量：

SELECT firstname 
FROM  person psn 
JOIN  ownership o ON psn.personid = o.owner 
JOIN  pet ON pet.petit = o.pet 
WHERE petname IN ('dog', 'cat') 
GROUP BY firstname 
HAVING COUNT(DISTINCT petname) = 2