删除空值查询

Question

我需要编写一个SQL查询，该查询将删除所有为空的书。

到目前为止，我已经创建了一个查询，该查询显示哪些书为空：

查询，显示哪些书为空

现在，我需要计算出SQL查询以删除所有已清空的书籍。我以为查询将是：

Answer 1

您可以在下面尝试

DELETE FROM BOOK
WHERE EXISTS (SELECT 1 FROM BOOK B 
  FULL JOIN CHECKOUT C ON C.BOOK_NUM = B.BOOK_NUM 
  WHERE C.CHECK_OUT_DATE IS NULL);

Answer 2

DELETE FROM BOOK 
 WHERE BOOK_NUM IN
(SELECT B.BOOK_NUM
  FROM BOOK B FULL JOIN CHECKOUT C ON C.BOOK_NUM = B.BOOK_NUM
 WHERE C.CHECK_OUT_DATE IS NULL);

内部查询将选择所有没有check_out_date的book_num。 如果内部查询返回book_num，则外部查询将删除books表中存在的书籍。

Answer 3

我不确定FULL JOIN在定义良好的数据模型中正在做什么。想必您想要：

DELETE FROM BOOK
WHERE NOT EXISTS (SELECT 1
                  FROM CHECKOUT C
                  WHERE C.BOOK_NUM = B.BOOK_NUM AND
                        C.CHECK_OUT_DATE IS NOT NULL
                 );