rails_vs_sql.rb

# SQL challenge #6
# find authors who have no surveys

# In Rails:
Survey.where(author_id :1).empty?
-OR-
a = Author.find(1)
a.surveys.empty?
a.surveys.any?


Rails vs SQL

Author.where(name: [fallon, jonathan, rocky, chris])
Author.where(id:  [1, 2, 3, 4 ]).map(&:name)


SELECT name FROM authors;
fallon
jonathan
rocky
chris


# Author.where in Rails   == SELECT * from authors
# Author.all.map(&:name)  == SELECT name from authors

1.
SELECT name
FROM authors
LEFT JOIN inner surveys
ON surveys.author_id=authors.id
WHERE surveys.author_id IS NULL;

2.
SELECT name, email, phone
FROM authors
WHERE id
NOT IN
  ( SELECT author_id FROM surveys );


()
SELECT name
FROM authors
WHERE id
IN
   ( SELECT id FROM surveys );