[gotski]

This was written on my mobile, so haven't had a chance to test it, but here's my first pass at modelling it in SQL:

  select sum( case when prev_cust.cust_id is null then 1 else 0 end) / sum( april_cust_count ) as pc_new_cust
  from (
    /* get unique customers in April */
    select distinct cust_id, 
        1 as april_cust_count
    from orders
    where order_date between date '2020-04-01' and date '2020-04-30'
  ) as april_cust
  left join
  (
    /* get customers with a transaction prior to April */
    select distinct cust_id
    from orders
    where order_date < date '2020-04-01'
  ) as prev_cust
  on april_cust.cust_id = 
  prev_cust.cust_id

Apologies for the lack of code formatting... I find that when SQL is written with a nice formatting (e.g. Nested sub queries with tabs) it reads a whole lot better.