1

DECODE(c1, 7, 'a', 
  6, 'b', 'c')

CASE c1
  WHEN 7 THEN 'a'
  WHEN 6 THEN 'b'
  ELSE 'c'
END

2

DECODE(c1, var1, 'a', 
  var2, 'b')

CASE
WHEN c1 = var1 OR 
    (c1 IS NULL AND 
    var1 IS NULL) THEN 'a'
WHEN c1 = var2 OR 
    (c1 IS NULL AND 
    var2 IS NULL) THEN 'b'
ELSE NULL
END

3

SELECT ID, DECODE(STATUS, 
     'A', 'Accepted',
     'D', 'Denied',
CAST(NULL AS VARCHAR(1)), 
   'Unknown', 'Other')
FROM CONTRACTS

SELECT ID,
CASE
WHEN STATUS = 'A' 
  THEN 'Accepted'
WHEN STATUS = 'D' 
  THEN 'Denied'
WHEN STATUS IS NULL 
  THEN 'Unknown'
ELSE 'Other'
END
FROM CONTRACTS