[sql-server] case in sql stored procedure on SQL Server

CASE isn't used for flow control... for this, you would need to use IF...

But, there's a set-based solution to this problem instead of the procedural approach:

UPDATE tblEmployee
SET 
  InOffice = CASE WHEN @NewStatus = 'InOffice' THEN -1 ELSE InOffice END,
  OutOffice = CASE WHEN @NewStatus = 'OutOffice' THEN -1 ELSE OutOffice END,
  Home = CASE WHEN @NewStatus = 'Home' THEN -1 ELSE Home END
WHERE EmpID = @EmpID

Note that the ELSE will preserves the original value if the @NewStatus condition isn't met.