How do you make a query that will default a value in the People Group field in Assignment?

Summary

We have a requirement such that when Job = Job123 and Payroll = Biweekly then default the People Group field to Crew. Is this possible?

Content

Navigation: Go to Manage Key Flexfield > Manage Structure Instances > Query for the flexfield > Click on Edit >Select a segment > Click on Edit > Select Default Type as SQL and write query in Default Value section

I need guidance on how the SQL should be written if the requirement is to set a default value in People Group when a certain Job and Payroll combination is assigned to an individual. 

Many thanks in advance.