SQL-包含空字段

Question

我不知道我是否正确表达了我的问题，但是我是SQL的新手，我真的不知道如何措辞。我正在查询有关作者及其所写书籍的信息，我想甚至包括那些有0本书的人。我该怎么做。这是我的代码，可以正常工作，但是遗漏了0本书：

SELECT surname as "Surname", forename as "Forename", COUNT(*) as "Books written"
FROM book
     LEFT OUTER JOIN author ON author.authorid = book.authorid
GROUP BY surname, forename
ORDER BY surname;

Answer 1

您从错误的级别开始。如果您希望所有作者都从作者开始。但是，由于您是从书本开始的，所以说给我所有符合我的标准的书，然后再加入作者。

SELECT surname as "Surname"
, forename as "Forename"
, COUNT(book.authorid) as "Books written"
   FROM author       
   LEFT OUTER JOIN book ON author.authorid = book.authorid
   GROUP BY surname, forename
   ORDER BY surname;

Answer 2

您应该反转选择的x联接，从作者中选择，然后联接其书籍。另外，您必须更改book.authorid属性的计数，因为这是您要计数的。

SELECT surname as "Surname", forename as "Forename", COUNT(book.authorid) as "Books written"
   FROM author 
   LEFT OUTER JOIN book ON author.authorid = book.authorid
   GROUP BY surname, forename
   ORDER BY surname;

这里的概念是，您要获取数据库中的所有作者，然后将其加入书中（甚至返回自从LEFT JOIN之后没有任何书的作者），而不是选择书，然后加入数据库作者。

有关SQL JOIN的说明，请参考以下图形表示

从DevMedia拍摄的图片

Answer 3

在count()内而不是count(*)内使用列名：

SELECT a.surname as "Surname", a.forename as "Forename", COUNT(b.authorid) as "Books written"
FROM book b LEFT OUTER JOIN 
     author a 
     ON a.authorid = b.authorid
GROUP BY a.surname, a.forename
ORDER BY a.surname;