????????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???  
    
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