Select case
When jdt_jty_code IN ('ISCO','ISSP','IECO','IECM','IESP','IEHD','ISHD','FRSI') Then 'Install'
When jdt_jty_code IN ('SB42','SB4W','HD42','HD4W','SA2C','SA2W','HD2C','HD2W','SNCO') Then 'Service Call'
Else 'UNKNOWN' END
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden