\u5728\u4f7f\u7528MySQL\u9032\u884c\u6578\u64da\u5eab\u7ba1\u7406\u6642\uff0c\u7a7a\u503c\uff08NULL\uff09\u662f\u4e00\u500b\u5e38\u898b\u7684\u6982\u5ff5\u3002\u96d6\u7136\u7a7a\u503c\u5728\u6578\u64da\u5eab\u4e2d\u6709\u5176\u5fc5\u8981\u6027\uff0c\u4f46\u5728\u8655\u7406\u7a7a\u503c\u6642\uff0c\u958b\u767c\u8005\u5e38\u5e38\u6703\u9047\u5230\u4e00\u4e9b\u9677\u9631\u3002\u672c\u6587\u5c07\u63a2\u8a0e\u5728MySQL\u4e2d\u8655\u7406\u7a7a\u503c\u6642\u9700\u8981\u7279\u5225\u6ce8\u610f\u7684\u5169\u500b\u4e3b\u8981\u9677\u9631\u3002<\/p>\n
\u5728MySQL\u4e2d\uff0c\u7a7a\u503c\uff08NULL\uff09\u548c\u96f6\uff080\uff09\u662f\u5169\u500b\u622a\u7136\u4e0d\u540c\u7684\u6982\u5ff5\u3002\u7a7a\u503c\u8868\u793a\u201c\u672a\u77e5\u201d\u6216\u201c\u7f3a\u5931\u201d\uff0c\u800c\u96f6\u5247\u662f\u4e00\u500b\u5177\u9ad4\u7684\u6578\u503c\u3002\u9019\u7a2e\u6df7\u6dc6\u53ef\u80fd\u5c0e\u81f4\u932f\u8aa4\u7684\u67e5\u8a62\u7d50\u679c\u6216\u6578\u64da\u8655\u7406\u3002<\/p>\n
\u4f8b\u5982\uff0c\u8003\u616e\u4ee5\u4e0b\u7684SQL\u67e5\u8a62\uff1a<\/p>\n
SELECT * FROM users WHERE age = 0;<\/code><\/pre>\n\u9019\u500b\u67e5\u8a62\u5c07\u8fd4\u56de\u6240\u6709\u5e74\u9f61\u70ba\u96f6\u7684\u7528\u6236\uff0c\u4f46\u5982\u679c\u6709\u7528\u6236\u7684\u5e74\u9f61\u70baNULL\uff0c\u9019\u4e9b\u7528\u6236\u5c07\u4e0d\u6703\u88ab\u5305\u542b\u5728\u7d50\u679c\u4e2d\u3002\u70ba\u4e86\u6b63\u78ba\u8655\u7406\u7a7a\u503c\uff0c\u958b\u767c\u8005\u61c9\u4f7f\u7528IS NULL\u4f86\u6aa2\u67e5\u7a7a\u503c\uff1a<\/p>\n
SELECT * FROM users WHERE age IS NULL;<\/code><\/pre>\n\u5728\u5be6\u969b\u61c9\u7528\u4e2d\uff0c\u958b\u767c\u8005\u61c9\u8a72\u660e\u78ba\u5340\u5206\u7a7a\u503c\u548c\u96f6\uff0c\u4e26\u6839\u64da\u5177\u9ad4\u9700\u6c42\u9078\u64c7\u5408\u9069\u7684\u67e5\u8a62\u689d\u4ef6\u3002<\/p>\n
\u9677\u9631\u4e8c\uff1a\u7a7a\u503c\u5728\u805a\u5408\u51fd\u6578\u4e2d\u7684\u884c\u70ba<\/h2>\n
\u53e6\u4e00\u500b\u5e38\u898b\u7684\u9677\u9631\u662f\u7a7a\u503c\u5728\u805a\u5408\u51fd\u6578\u4e2d\u7684\u884c\u70ba\u3002MySQL\u7684\u805a\u5408\u51fd\u6578\uff08\u5982COUNT\u3001SUM\u3001AVG\u7b49\uff09\u5728\u8655\u7406\u7a7a\u503c\u6642\u6703\u6709\u7279\u5b9a\u7684\u898f\u5247\u3002\u4f8b\u5982\uff0cCOUNT\u51fd\u6578\u8a08\u7b97\u975e\u7a7a\u503c\u7684\u6578\u91cf\uff0c\u800cSUM\u548cAVG\u5247\u6703\u5ffd\u7565\u7a7a\u503c\u3002<\/p>\n
\u8003\u616e\u4ee5\u4e0b\u7684\u6578\u64da\u8868\uff1a<\/p>\n
CREATE TABLE sales (\n id INT,\n amount DECIMAL(10, 2)\n);\n\nINSERT INTO sales (id, amount) VALUES (1, 100.00), (2, NULL), (3, 200.00);\n<\/code><\/pre>\n\u5982\u679c\u6211\u5011\u57f7\u884c\u4ee5\u4e0b\u67e5\u8a62\uff1a<\/p>\n
SELECT COUNT(amount) FROM sales;<\/code><\/pre>\n\u9019\u5c07\u8fd4\u56de2\uff0c\u56e0\u70ba\u53ea\u6709\u5169\u500b\u975e\u7a7a\u503c\u88ab\u8a08\u7b97\u5728\u5167\u3002\u7136\u800c\uff0c\u5982\u679c\u6211\u5011\u60f3\u8a08\u7b97\u7e3d\u92b7\u552e\u984d\uff0c\u5247\u53ef\u4ee5\u4f7f\u7528\uff1a<\/p>\n
SELECT SUM(amount) FROM sales;<\/code><\/pre>\n\u9019\u5c07\u8fd4\u56de300.00\uff0c\u56e0\u70baNULL\u503c\u88ab\u81ea\u52d5\u5ffd\u7565\u3002\u9019\u7a2e\u884c\u70ba\u53ef\u80fd\u6703\u5c0e\u81f4\u958b\u767c\u8005\u5728\u8a08\u7b97\u6642\u51fa\u73fe\u610f\u5916\u7684\u7d50\u679c\uff0c\u56e0\u6b64\u5728\u4f7f\u7528\u805a\u5408\u51fd\u6578\u6642\uff0c\u5fc5\u9808\u7279\u5225\u6ce8\u610f\u7a7a\u503c\u7684\u5f71\u97ff\u3002<\/p>\n
\u7d50\u8ad6<\/h2>\n
\u5728MySQL\u4e2d\u8655\u7406\u7a7a\u503c\u6642\uff0c\u958b\u767c\u8005\u9700\u8981\u5c0f\u5fc3\u5169\u500b\u4e3b\u8981\u9677\u9631\uff1a\u7a7a\u503c\u8207\u96f6\u7684\u6df7\u6dc6\uff0c\u4ee5\u53ca\u7a7a\u503c\u5728\u805a\u5408\u51fd\u6578\u4e2d\u7684\u884c\u70ba\u3002\u4e86\u89e3\u9019\u4e9b\u9677\u9631\u53ef\u4ee5\u5e6b\u52a9\u958b\u767c\u8005\u66f4\u6709\u6548\u5730\u7ba1\u7406\u6578\u64da\uff0c\u907f\u514d\u6f5b\u5728\u7684\u932f\u8aa4\u3002<\/p>\n
\u5c0d\u65bc\u9700\u8981\u7a69\u5b9a\u548c\u9ad8\u6548\u6578\u64da\u5eab\u89e3\u6c7a\u65b9\u6848\u7684\u7528\u6236\uff0c\u9078\u64c7\u5408\u9069\u7684 VPS<\/a> \u6216 \u9999\u6e2f\u4f3a\u670d\u5668<\/a> \u662f\u81f3\u95dc\u91cd\u8981\u7684\u3002\u7121\u8ad6\u662f\u6578\u64da\u5eab\u7ba1\u7406\u9084\u662f\u5176\u4ed6\u61c9\u7528\uff0c\u9078\u64c7\u4e00\u500b\u53ef\u9760\u7684\u670d\u52d9\u63d0\u4f9b\u5546\u5c07\u6709\u52a9\u65bc\u78ba\u4fdd\u7cfb\u7d71\u7684\u7a69\u5b9a\u6027\u548c\u6027\u80fd\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"\u4e86\u89e3MySQL\u4e2d\u8655\u7406\u7a7a\u503c\u7684\u5169\u500b\u5e38\u898b\u9677\u9631\uff0c\u907f\u514d\u6578\u64da\u932f\u8aa4\u548c\u67e5\u8a62\u554f\u984c\uff0c\u63d0\u5347\u6578\u64da\u5eab\u64cd\u4f5c\u7684\u6e96\u78ba\u6027\u548c\u6548\u7387\u3002<\/p>\n","protected":false},"author":0,"featured_media":0,"comment_status":"closed","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[101],"tags":[],"class_list":["post-80343","post","type-post","status-publish","format-standard","hentry","category-database"],"_links":{"self":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/80343","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/types\/post"}],"replies":[{"embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/comments?post=80343"}],"version-history":[{"count":1,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/80343\/revisions"}],"predecessor-version":[{"id":80344,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/80343\/revisions\/80344"}],"wp:attachment":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/media?parent=80343"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/categories?post=80343"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/tags?post=80343"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}