Question

我有一个名为“书籍”的表格，其中有很多“章节”，我希望得到所有超过10章的书籍。如何在单个查询中执行此操作？

到目前为止，我有这个......

Books.joins('LEFT JOIN chapters ON chapters.book_id = books.id')

Answer 1

以下是使用Rails 4，ActiveRecord

的查询

Book.includes(:chapters).references(:chapters).group('books.id').having('count(chapters.id) > 10')

Answer 2

我不太了解rails，但SQL会是这样的：

SELECT b.* FROM Books b 
JOIN Chapters c ON b.id = c.book_id 
GROUP BY b.id
HAVING COUNT(*) > 10

根据ActiveRecord文档，它将是

Books.joins(:chapters)
  .group('books.id')
  .having('count() > 10')

更新：我明白了。这里我在rails中做了什么，如果它可以帮助任何人

query = <<-SQL
  UPDATE books AS b
  INNER JOIN
  (
      SELECT books.id
      FROM books
      JOIN chapters ON chapters.book_id = books.id
      GROUP BY books.id
      HAVING count(chapters.id) > 10
  ) i
  ON b.id = i.id
  SET long_book = true;
SQL

ActiveRecord::Base.connection.execute(query)

Answer 3

找到解决方案......

query = <<-SQL
  UPDATE books AS b
  INNER JOIN
  (
      SELECT books.id
      FROM books
      JOIN chapters ON chapters.book_id = books.id
      GROUP BY books.id
      HAVING count(chapters.id) > 10
  ) i
  ON b.id = i.id
  SET long_book = true;
SQL

ActiveRecord::Base.connection.execute(query)

Rails SQL查询计数

3 个答案: