Question

问题12.找出拥有或借用该书“978-”的学生的姓名。 0262033848’ 。使用 UNION 。

select s.name
from student s, loan l
where s.email = l.borrower and l.book = '978-0262033848'
union
select s.name
from student s, copy c
where s.email = c.owner and c.book = '978-0262033848';

问题13.找出拥有或借用该书“978-”的学生的姓名。 0262033848’ 。使用 OR 。

我该怎么做？

Answer 1

您可以将这三个表连接在一起，然后在OR子句中使用WHERE条件来获得所需的结果集：

SELECT DISTINCT s.name
FROM student s
LEFT JOIN loan l
    ON s.email = l.borrower
LEFT JOIN copy c
    ON s.email = c.owner
WHERE l.book = '978-0262033848' OR
      c.book = '978-0262033848';

您还会注意到我使用显式左连接替换了您的隐式连接，将连接条件从WHERE移动到ON子句。这是最佳做法，因为它使您的查询更易于阅读和维护。

<强>更新

我给你的查询是这样做的首选方式。如果你想坚持使用ANSI-92之前的表格，你可以试试这个：

SELECT DISTINCT s.name
FROM student s, loan l, copy c
WHERE s.email = l.borrower OR
      s.email = c.owner AND
      (l.book = '978-0262033848' OR
       c.book = '978-0262033848')

Answer 2

select s.name
from student s
left join loan as l on s.email = l.borrower
left join copy as c on s.email = c.owner
where l.book = '978-0262033848' or c.book = '978-0262033848'

将联盟更改为OR

2 个答案: