sqlÖÐÔõÑù½«Îª¿Õ±äΪ0
ÔÚ sql ÖУ¬¿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×㣬Ôò·µ»ØĬÈÏÖµ 0¡£
ÔõÑù½« SQL ÖÐµÄ NULL ֵת»»Îª 0
ÔÚ SQL ÅÌÎÊÖУ¬½« NULL ֵת»»Îª 0 ·Ç¾³£¼û¡£¿ÉÒÔʹÓÃÒÔÏÂÒªÁìÖ®Ò»£º
1. COALESCE º¯Êý:
COALESCE º¯ÊýʹÓÃÒÔÏÂÓï·¨£º
COALESCE(expression1, expression2, ..., expressionN)
µÇ¼ºó¸´ÖÆ
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;
µÇ¼ºó¸´ÖÆ
Õ⽫Óà 0 Ìæ»»ËùÓпÕÖµ¡£
2. ISNULL º¯Êý:
ISNULL º¯ÊýʹÓÃÒÔÏÂÓï·¨£º
ISNULL(expression, replacement_value)
µÇ¼ºó¸´ÖÆ
ÆäÖУº
expression ÊÇÒª¼ì²éµÄ±í´ïʽ¡£
replacement_value ÊÇÈôÊÇ expression Ϊ NULL ʱҪ·µ»ØµÄÖµ¡£
ʾÀý£º
SELECT ISNULL(salary, 0) AS updated_salary FROM employees;
µÇ¼ºó¸´ÖÆ
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
µÇ¼ºó¸´ÖÆ
ÆäÖУº
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¡£
ʾÀý£º
SELECT CASE WHEN salary IS NULL THEN 0 ELSE salary END AS updated_salary FROM employees;
µÇ¼ºó¸´ÖÆ
Õ⽫¼ì²é salary ÁÐÊÇ·ñΪ NULL£¬ÈôÊÇÊÇ£¬Ôò·µ»Ø 0£¬²»È»·µ»Ø salary Öµ¡£
ÒÔÉϾÍÊÇsqlÖÐÔõÑù½«Îª¿Õ±äΪ0µÄÏêϸÄÚÈÝ£¬¸ü¶àÇë¹Ø×¢±¾ÍøÄÚÆäËüÏà¹ØÎÄÕ£¡