sql case in from clause code example

Example 1: case statement in sql

Case Statement basically
Like IF - THEN - ELSE statement.

The CASE statement goes through conditions
and returns a value when the
first condition is met and
once a condition is true,
it will stop reading and return the result.
If no conditions are true,
it returns the value in the ELSE clause.

If there is no ELSE part and
no conditions are true, it returns NULL.

FOR EXAMPLE =

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    WHEN conditionN THEN resultN
    ELSE result
END 

-- example:
SELECT 
	CASE
      WHEN (1+6 = 6) THEN 'A'
      WHEN (1+6 = 7) THEN 'B'
      WHEN (1+6 = 8) THEN 'C'
      ELSE 'D'
	END 
FROM DUAL;

Result would be 'B' since it is the first
correct answer

Example 2: 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