为了说明我的问题,考虑以下三个表:<标题>:
personid int auto_increment not null,
firstname varchar(16) not null,
constraint pk_person primary key (personid)
<标题>宠物:petid int auto_increment not null,
petname varchar(16) not null,
constraint pk_pet primary key (petid)
<标题>所有权: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)
和元组:
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
我想要一个只返回拥有狗和猫的主人的查询,在本例中是carly。宠物的数目可以多于这两个
标题>标题>标题>有几种方法可以做到这一点,包括使用两个exists
条件。不过,我个人最喜欢的是查询哪些主人养猫或狗,并计算他们养宠物的不同数量:
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