????????Oracle????????????????????????????????????????????????????????????????????????????

NVL?NON-VALIDating???
      
NVL??????????????????????????NVL(expr1, expr2)???expr1?????????expr2???????????NVL?????????????????????????
      
???
      
SELECT NVL(null, 'ok') FROM dual; -- ???'ok'
      
SELECT NVL('something', 'ok') FROM dual; -- ???'something'
      
NVL2??
      
NVL2???NVL???????????????NVL2(expr1, expr2, expr3)???expr1?????expr2???expr1??????expr3?NVL2?????????????????
      
???
      
SELECT NVL2(null, 'ok1', 'ok2') FROM dual; -- ???'ok2'
      
SELECT NVL2('something', 'ok1', 'ok2') FROM dual; -- ???'ok1'
      
NULLIF??
      
NULLIF?????????????????NULL????????????????????NULLIF(exp1, exp2)???exp1??exp2???NULL??????exp1???
      
???
      
SELECT NULLIF(1, 1) FROM dual; -- ???''
      
SELECT NULLIF(1, 2) FROM dual; -- ???'1'
      
COALESCE??
      
COALESCE??????????????????????????????????NULL??????????????????????
      
???
      
SELECT COALESCE(null, 1, 2, 3) FROM dual; -- ???'1'
      
SELECT COALESCE(null, null, 2, 3) FROM dual; -- ???'2'
      
SELECT COALESCE(null, null, null, 3) FROM dual; -- ???'3'
      
SELECT COALESCE(null, 1, null, 2) FROM dual; -- ???'1'
      
?????
      - ????????????????NVL???      - ??????????????????????????????COALESCE???      - ?????????????????NULLIF???      - ???????????????????NVL2???      
???????????????????????????????????????????????????????????????????