Parses a csv to look for email addresses. Then checks the sql table to see which emails exist and then returns those users ids.

parse_csv_check_sql.rb

require 'mysql2'
require 'csv'

# Returns the user ids for emails that already exist
class Users
  CSV_FILENAME = 'emails.csv'.freeze
  EMAIL_COLUMN_NUMBER = 0

  def user_ids
    ids = []
    results = client.query(
      'SELECT id, first_name, last_name, email
       FROM users WHERE email IN (' + emails.join(', ') + ')'
    )

    results.each do |row|
      ids.push(row['id'])
    end

    puts ids.join(', ') # return comma separated ids
  end

  def emails
    @emails = []
    CSV.foreach(CSV_FILENAME) do |row|
      @emails.push(row[EMAIL_COLUMN_NUMBER])
    end
    @emails # return the array
  end

  def client
    @client ||= Mysql2::Client.new(
      host:     'localhost',
      username: 'root',
      database: 'test_database'
    )
  end
end

Users.new.user_ids