Suppose the following tables are part of a project management database of a company , wher e keys are bold: E ( ssn , name , salary ) stores the information of employees , P ( pid , title , budget ) stores the information of projects, and M ( ssn , pid , start, end ) stores the information of management of projects by employees .

Write an SQL query to return the ssns of employees that managed at most 1 project completed or ongoing in last 6 months.