什么是正确的SQL语句来实现以下结果

Question

我有下表。

Owner_id    Owner
-----------------
1           Bill
2           Steve


Animal      Owner_fk_id
-----------------------
Cat         1
Dog         1
Lion        2

Car         Owner_fk_id
-----------------------
Ferrari     1
BMW         1
Lotus       2

我希望列出所有动物和汽车，对于拥有至少1辆法拉利车主的车主。由于比尔拥有法拉利，我们将列出所有比尔的汽车和动物。

如果我使用以下SQL：

SELECT *
    FROM Owner
    LEFT JOIN Animal ON (Animal.Owner_fk_id = Owner.Owner_id )
    LEFT JOIN Car ON (Car.Owner_fk_id = Owner.Owner_id )
    WHERE Car.Car = 'Ferrari'

我将会

Owner       Animal  Car
---------------------------
Bill        Cat     Ferrari
Bill        Dog     Ferrari

我希望的结果是什么

Owner       Animal  Car
---------------------------
Bill        Cat     Ferrari
Bill        Cat     BMW
Bill        Dog     Ferrari
Bill        Dog     BMW

我可以知道我应该使用的正确SQL语句是什么吗？

Answer 1

根据您所需的条件进行内部联接（即Car='Ferrari），然后像以前一样加入其他表格。

SELECT o.Owner, a.Animal, c2.Car
FROM Owner as o
INNER JOIN Car as c1 on c1.Owner_fk_id = o.Owner_id and c1.Car='Ferrari'
LEFT JOIN Animal as a ON a.Owner_fk_id = o.Owner_id
LEFT JOIN Car as c2 ON c2.Owner_fk_id = o.Owner_id;

Answer 2

如果一个Owner拥有多辆法拉利而且只计算一次他/她很重要，你可以这样做：

SELECT subq.Owner, a.Animal, c2.Car FROM
    (SELECT o.* FROM Owner as o1 WHERE EXISTS
        (SELECT 1 FROM Car as c1
         WHERE c1.Owner_fk_id = o1.Owner_id
         AND c1.Car='Ferrari')
    ) as subq
LEFT JOIN Animal as a ON a.Owner_fk_id = subq.Owner_id
LEFT JOIN Car as c2 ON c2.Owner_fk_id = subq.Owner_id;