How to identify the operation type(insert,update,delete) in SQL Server trigger

Question

We are using the following trigger in SQL Server to maintain the history now I need to identify the operations just like insert,update or delete. I found some information HERE but it doesn't works with the SQL Server.

CREATE TRIGGER audit_guest_details ON [PMS].[GSDTLTBL] 
FOR INSERT,UPDATE,DELETE
AS
    DECLARE @SRLNUB1 INT;
    DECLARE @UPDFLG1 DECIMAL(3,0);

    SELECT @SRLNUB1 = I.SRLNUB FROM inserted I;
    SELECT @UPDFLG1 = I.UPDFLG FROM inserted I;   

    BEGIN
       /* Here I need to identify the operation and insert the operation type in the GUEST_ADT 3rd field */
       insert into dbo.GUEST_ADT values(@SRLNUB1,@UPDFLG1,?);

       PRINT 'BEFORE INSERT trigger fired.'
    END;
GO

But here I need to identify the operation and want to insert operation type accordingly.

Here I don't want to create three trigger for every operations

Solution 1

For Inserted : Rows are in inserted only. For Updated: Rows are in inserted and deleted. For Deleted: Rows are in deleted only.

DECLARE @event_type varchar(42)
IF EXISTS(SELECT * FROM inserted)
  IF EXISTS(SELECT * FROM deleted)
    SELECT @event_type = 'update'
ELSE
    SELECT @event_type = 'insert'
ELSE
  IF EXISTS(SELECT * FROM deleted)
    SELECT @event_type = 'delete'
  ELSE
    --no rows affected - cannot determine event
    SELECT @event_type = 'unknown'

Solution 2

This is a simplified version of Mikhail's answer that uses a searched CASE expression.

DECLARE @Operation varchar(7) = 
    CASE WHEN EXISTS(SELECT * FROM inserted) AND EXISTS(SELECT * FROM deleted) 
        THEN 'Update'
    WHEN EXISTS(SELECT * FROM inserted) 
        THEN 'Insert'
    WHEN EXISTS(SELECT * FROM deleted)
        THEN 'Delete'
    ELSE 
        NULL --Unknown
    END;