text: // Hi, I'm Yanzhan. I'm interested in all kinds of algorithmic problems. Also, I'm fascinated with javascript. If you want to learn more about programming problems and javascript, please visit my Youtube Channel (https://www.youtube.com/channel/UCDkz-__gl3frqLexukpG0DA?view_as=subscriber), Twitter Account (https://twitter.com/YangYanzhan), GitHub Repo (https://github.com/yangyanzhan/code-camp) or HomePage (https://yanzhan.site/).
  
  // 开始分析
  // 1. SQL。
  // 结束分析
  
  // 开始编码
  
  select customer_id,count(if(transaction_id is null,1,null))as count_no_trans
  from visits
  left join
  transactions
  using(visit_id)
  group by customer_id
  having count_no_trans>0;
  