Finding/searching for missing values in MySQL

Question

I am using MySQL and have a table called sales. Its primary key is sales_id.

-------------------------------------
sales_id | invo         


        

Answer 1

set @suggest_invoice:=147;

select 
  sales_id, 
  invoice_id,
  @suggest_invoice:=@suggest_invoice+1,
  if(invoice_id=@suggest_invoice, 0, 1) as missing_invoice
from sales
order by sales_id;

I guess you almost getting all the records except the first two with missing_invoice=1