title: Knex.js migrate data from one column to others conditionally
body: <p>I'm trying to do a migration with knex.js in a dirty MySQL database.
  So there is one column in which there is JSON with three different types of data.
  I already made a migration in which I created the three different columns for these different types of data.</p>
  
  <p>What I now want to do, is conditionally move the the data from the column with the JSON to the three different columns so:
  <code>
  if type == A:
      move to new_col_A
  else if type == B:
      move to new_col_B
  else:
      move to new_col_C
  </code>
  So eventually this column with the JSON should be empty and the three new columns should be populated with the data that was in this JSON column.</p>