尊龙凯时人生就是搏

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ÔÚ sql ÖÐ £¬¿ÉÒÔʹÓà coalesce¡¢isnull »ò case ±í´ïʽ½« null ֵת»»Îª 0£ºcoalesce º¯Êý¼ì²é¶à¸ö±í´ïʽ £¬·µ»ØµÚÒ»¸ö·Ç null Öµ £¬ÈôÊÇËùÓбí´ïʽ¾ùΪ null £¬Ôò·µ»ØÖ¸¶¨µÄĬÈÏÖµ 0¡£isnull º¯Êý¼ì²éÒ»¸ö±í´ïʽ £¬ÈôÊÇΪ null £¬Ôò·µ»ØÖ¸¶¨µÄĬÈÏÖµ 0¡£case ±í´ïʽƾ֤Ìõ¼þÆÀ¹À±í´ïʽ £¬·µ»ØÖ¸¶¨µÄÖµ £¬ÈôÊÇÌõ¼þ²»Öª×ã £¬Ôò·µ»ØĬÈÏÖµ 0¡£

ÔõÑù½« SQL ÖÐµÄ NULL ֵת»»Îª 0

ÔÚ SQL ÅÌÎÊÖÐ £¬½« NULL ֵת»»Îª 0 ·Ç¾­³£¼û¡£¿ÉÒÔʹÓÃÒÔÏÂÒªÁìÖ®Ò»£º

1. COALESCE º¯Êý:

COALESCE º¯ÊýʹÓÃÒÔÏÂÓï·¨£º

COALESCE(expression1, expression2, ..., expressionN)

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ÆäÖУº

expression1 ÊÇÒª¼ì²éµÄµÚÒ»¸ö±í´ïʽ¡£

expression2 £¬… £¬expressionN ÊÇ¿ÉÑ¡µÄ±¸Óñí´ïʽ¡£

ÈôÊÇ expression1 ²»Îª NULL £¬Ôò COALESCE ·µ»Ø expression1£»²»È» £¬Ëü·µ»ØµÚÒ»¸ö²»Îª NULL µÄ±¸Óñí´ïʽ£¨ÈôÊÇÓУ©¡£

ʾÀý£º

SELECT COALESCE(salary, 0) AS updated_salary
FROM employees;

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Õ⽫Óà 0 Ìæ»»ËùÓпÕÖµ¡£

2. ISNULL º¯Êý:

ISNULL º¯ÊýʹÓÃÒÔÏÂÓï·¨£º

ISNULL(expression, replacement_value)

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expression ÊÇÒª¼ì²éµÄ±í´ïʽ¡£

replacement_value ÊÇÈôÊÇ expression Ϊ NULL ʱҪ·µ»ØµÄÖµ¡£

ʾÀý£º

SELECT ISNULL(salary, 0) AS updated_salary
FROM employees;

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ÕâÓë COALESCE º¯ÊýµÄÊÂÇé·½·¨ÀàËÆ¡£

3. CASE ±í´ïʽ:

CASE ±í´ïʽʹÓÃÒÔÏÂÓï·¨£º

CASE
  WHEN expression = value THEN result_1
  WHEN ...
  ELSE result_N
END

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expression ÊÇÒªÆÀ¹ÀµÄ±í´ïʽ¡£

value ÊÇÒª¼ì²éµÄÖµ¡£

result_1 ÊÇÈôÊÇ expression ¼´ÊÇ value ʱҪ·µ»ØµÄÖµ¡£

… £¬resultN ÊÇ¿ÉÑ¡µÄ±¸ÓÃЧ¹û¡£

ELSE ÊÇ¿ÉÑ¡µÄ £¬ÓÃÓÚ·µ»ØÈôÊÇ expression ÓëÈκΠvalue ²»Æ¥ÅäʱµÄĬÈÏÖµ¡£

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SELECT CASE
  WHEN salary IS NULL THEN 0
  ELSE salary
END AS updated_salary
FROM employees;

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Õ⽫¼ì²é salary ÁÐÊÇ·ñΪ NULL £¬ÈôÊÇÊÇ £¬Ôò·µ»Ø 0 £¬²»È»·µ»Ø salary Öµ¡£

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