title: How to count monthly retention user in bigquery?
body: <p>I have raw data as below. With each line is the record of an transaction of user, and the month when they made the transaction <br/>
  <a href="https://i.stack.imgur.com/TRFMk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TRFMk.png" alt="enter image description here"></a></p>
  
  <p><br/>
  What I want is to calculate the number of user who made order in a month and the number of repeated user (RETENTION) from last month, then I can know how many % of user is repeated user.</p>
  
  <p>The desired result should look like this 
  <a href="https://i.stack.imgur.com/kaBo1.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kaBo1.png" alt="enter image description here"></a> <br/></p>
  
  <p>How can I do it in bigquery?</p>