ActiveRecord/SQL: 复杂的数据库查询 (outer_join / union / ?)

Question

数据库查询：我想对所有参加培训的参与者进行一次数据库查询，显示所有参与者的姓名以及他们是否已经拥有一项特定技能。为了构建结果 table（参见示例），我想遍历查询结果，而不必为每个员工进行子查询。

|participant name|has_this_one_specific_skill|
|John Doe        |          yes              |
|Alex Hardacre   |          yes              |
|Abi Underwood   |          no               |
|Kevin Wallace   |          yes              |
|Connor Murray   |          no               |

这是我的数据库结构。这样的结构会是什么样子？这是 left_outer_join 还是我要加入联盟？

Answer 1

class Employee < ApplicationRecord
  has_many :participants
  has_many :trainings,through: :participants
  has_many :trained_skills,through: :trainings,source: :skill
end



class Training < ApplicationRecord
  belongs_to :skill
end


class Participant < ApplicationRecord
  belongs_to :training
  belongs_to :employee
end

解决方案 1 - 推荐

   Employee.left_joins(:trained_skills).where({skills: {name: ["MYSQL",nil]}}).pluck "employees.name,CASE WHEN skills.id IS NOT NULL THEN 'yes' else 'no' end"

注意：如果您需要 Employees 对象，您可以使用 select 而不是 pluck。即######.select "employees.name as participant_name,CASE WHEN skills.id THEN 'yes' else 'no' end as has_this_one_specific_skill"

解决方案 2

 Employee.joins("LEFT OUTER JOIN participants
  ON participants.employee_id = employees.id LEFT OUTER JOIN trainings
  ON trainings.id = participants.training_id LEFT OUTER JOIN skills
  ON skills.id = trainings.skill_id AND skills.name = 'MYSQL'").
  pluck "employees.name,CASE WHEN skills.id IS NOT NULL THEN 'yes' else 'no' end"

ActiveRecord/SQL: 复杂的数据库查询 (outer_join / union / ?)

ActiveRecord/SQL: Complex DB query (outer_join / union / ?)

ruby

sql

activerecord

ruby-on-rails