update with case statement in sql code example

Example 1: case when switch in SQL

-- Case Eg.) to retrive the MAX value of a Field 
-- if there are entries for the Field in table MAX value will be returned 
-- But if there is no entries at all for the Field in tabel MAX will return
-- Null as the output. But Using Case When we can check it out return zero 
-- or any other value if there is no enties for the Field in table..
SELECT 
CASE   -- Like Switch Case
	WHEN -- First When condition 
		(MAX(BILLID) IS NULL) -- Condition 
	THEN 1   -- output   (We can also add more When conditions like Above)
ELSE -- When WHEN Condition not Satisfied Below will be Executed. 
		(MAX(BILLID)) -- output
END 
as MAXBILLID   from  DUAL;
-- Final Output
-- If there is no entry in the Field for the table
-- BILLID
--  1
-- If there are entries MAX of that Field value from the table
-- BILLID
-- 10

Example 2: sql case update

UPDATE dbo.TestStudents  
SET     LASTNAME =  CASE  
                        WHEN LASTNAME = 'AAA' THEN 'BBB' 
                        WHEN LASTNAME = 'CCC' THEN 'DDD' 
                        WHEN LASTNAME = 'EEE' THEN 'FFF' 
                        ELSE LASTNAME
                    END 
WHERE   LASTNAME IN ('AAA', 'CCC', 'EEE')