{"id":173301,"date":"2024-11-07T16:53:38","date_gmt":"2024-11-07T08:53:38","guid":{"rendered":"https:\/\/server.hk\/cnblog\/173301\/"},"modified":"2024-11-07T16:53:39","modified_gmt":"2024-11-07T08:53:39","slug":"%e5%a6%82%e4%bd%95%e5%ae%9a%e7%be%a9%e6%95%b8%e6%93%9a%e5%ba%ab%e4%b8%ad%e7%9a%84null%e5%80%bc%ef%bc%9f-%e6%95%b8%e6%93%9a%e5%ba%ab%e6%80%8e%e9%ba%bc%e5%ae%9a%e7%be%a9null%e5%80%bc","status":"publish","type":"post","link":"https:\/\/server.hk\/cnblog\/173301\/","title":{"rendered":"\u5982\u4f55\u5b9a\u7fa9\u6578\u64da\u5eab\u4e2d\u7684null\u503c\uff1f (\u6578\u64da\u5eab\u600e\u9ebc\u5b9a\u7fa9null\u503c)"},"content":{"rendered":"<h1 id=\"%e5%a6%82%e4%bd%95%e5%ae%9a%e7%be%a9%e6%95%b8%e6%93%9a%e5%ba%ab%e4%b8%ad%e7%9a%84null%e5%80%bc%ef%bc%9f-FyLfKWlBeW\">\u5982\u4f55\u5b9a\u7fa9\u6578\u64da\u5eab\u4e2d\u7684null\u503c\uff1f<\/h1>\n<p>\u5728\u6578\u64da\u5eab\u7ba1\u7406\u7cfb\u7d71\u4e2d\uff0cnull\u503c\u662f\u4e00\u500b\u91cd\u8981\u7684\u6982\u5ff5\uff0c\u5c0d\u65bc\u6578\u64da\u7684\u5b8c\u6574\u6027\u548c\u6e96\u78ba\u6027\u6709\u8457\u6df1\u9060\u7684\u5f71\u97ff\u3002\u672c\u6587\u5c07\u63a2\u8a0enull\u503c\u7684\u5b9a\u7fa9\u3001\u7279\u6027\u4ee5\u53ca\u5728\u6578\u64da\u5eab\u4e2d\u7684\u61c9\u7528\uff0c\u5e6b\u52a9\u8b80\u8005\u66f4\u597d\u5730\u7406\u89e3\u9019\u4e00\u6982\u5ff5\u3002<\/p>\n<h2 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id=\"null%e5%80%bc%e7%9a%84%e7%89%b9%e6%80%a7-FyLfKWlBeW\">null\u503c\u7684\u7279\u6027<\/h2>\n<ul>\n<li><strong>\u4e0d\u7b49\u65bc\u4efb\u4f55\u503c\uff1a<\/strong>\u5728\u6578\u64da\u5eab\u4e2d\uff0cnull\u503c\u4e0d\u7b49\u65bc\u4efb\u4f55\u5176\u4ed6\u503c\uff0c\u5305\u62ec\u81ea\u8eab\u3002\u9019\u610f\u5473\u8457\u5982\u679c\u4f60\u6bd4\u8f03\u4e00\u500b\u5b57\u6bb5\u7684\u503c\u8207null\uff0c\u7d50\u679c\u5c07\u662ffalse\u3002<\/li>\n<li><strong>\u4e0d\u5f71\u97ff\u6578\u64da\u985e\u578b\uff1a<\/strong>\u5373\u4f7f\u4e00\u500b\u5b57\u6bb5\u7684\u503c\u70banull\uff0c\u5b83\u4ecd\u7136\u4fdd\u6301\u5176\u6578\u64da\u985e\u578b\u3002\u4f8b\u5982\uff0c\u4e00\u500b\u6574\u6578\u985e\u578b\u7684\u5b57\u6bb5\u53ef\u4ee5\u662fnull\uff0c\u9019\u4e26\u4e0d\u6539\u8b8a\u5b83\u7684\u6574\u6578\u985e\u578b\u3002<\/li>\n<li><strong>\u53ef\u7528\u65bc\u8868\u793a\u7f3a\u5931\u6578\u64da\uff1a<\/strong>null\u503c\u5e38\u7528\u65bc\u8868\u793a\u6578\u64da\u7684\u7f3a\u5931\u6216\u672a\u77e5\uff0c\u9019\u5728\u6578\u64da\u6536\u96c6\u548c\u5206\u6790\u4e2d\u975e\u5e38\u91cd\u8981\u3002<\/li>\n<\/ul>\n<h2 id=\"%e5%a6%82%e4%bd%95%e5%9c%a8%e6%95%b8%e6%93%9a%e5%ba%ab%e4%b8%ad%e4%bd%bf%e7%94%a8null%e5%80%bc-FyLfKWlBeW\">\u5982\u4f55\u5728\u6578\u64da\u5eab\u4e2d\u4f7f\u7528null\u503c<\/h2>\n<p>\u5728\u6578\u64da\u5eab\u8a2d\u8a08\u4e2d\uff0cnull\u503c\u7684\u4f7f\u7528\u9700\u8981\u8b39\u614e\u8003\u616e\u3002\u4ee5\u4e0b\u662f\u4e00\u4e9b\u5e38\u898b\u7684\u4f7f\u7528\u5834\u666f\uff1a<\/p>\n<h3 id=\"1-%e8%a8%ad%e8%a8%88%e6%95%b8%e6%93%9a%e8%a1%a8-FyLfKWlBeW\">1. \u8a2d\u8a08\u6578\u64da\u8868<\/h3>\n<p>\u5728\u5275\u5efa\u6578\u64da\u8868\u6642\uff0c\u53ef\u4ee5\u6307\u5b9a\u67d0\u4e9b\u5b57\u6bb5\u5141\u8a31null\u503c\u3002\u4f8b\u5982\uff1a<\/p>\n<pre><code>CREATE TABLE Users (\n    UserID INT PRIMARY KEY,\n    UserName VARCHAR(100) NOT NULL,\n    Email VARCHAR(100) NULL,\n    PhoneNumber VARCHAR(15) NULL\n);<\/code><\/pre>\n<p>\u5728\u9019\u500b\u4f8b\u5b50\u4e2d\uff0cUserName\u5b57\u6bb5\u4e0d\u5141\u8a31\u70banull\uff0c\u800cEmail\u548cPhoneNumber\u5b57\u6bb5\u5247\u53ef\u4ee5\u70banull\uff0c\u8868\u793a\u9019\u4e9b\u4fe1\u606f\u53ef\u80fd\u4e0d\u9069\u7528\u6216\u672a\u77e5\u3002<\/p>\n<h3 id=\"2-%e6%9f%a5%e8%a9%a2%e6%95%b8%e6%93%9a-FyLfKWlBeW\">2. \u67e5\u8a62\u6578\u64da<\/h3>\n<p>\u5728\u67e5\u8a62\u6578\u64da\u6642\uff0c\u53ef\u4ee5\u4f7f\u7528IS NULL\u6216IS NOT NULL\u4f86\u6aa2\u67e5null\u503c\u3002\u4f8b\u5982\uff1a<\/p>\n<pre><code>SELECT * FROM Users WHERE Email IS NULL;<\/code><\/pre>\n<p>\u9019\u689d\u67e5\u8a62\u5c07\u8fd4\u56de\u6240\u6709Email\u5b57\u6bb5\u70banull\u7684\u7528\u6236\u3002<\/p>\n<h3 id=\"3-%e6%95%b8%e6%93%9a%e5%ae%8c%e6%95%b4%e6%80%a7-FyLfKWlBeW\">3. \u6578\u64da\u5b8c\u6574\u6027<\/h3>\n<p>\u5728\u6578\u64da\u5eab\u4e2d\uff0cnull\u503c\u7684\u4f7f\u7528\u53ef\u4ee5\u5e6b\u52a9\u7dad\u8b77\u6578\u64da\u7684\u5b8c\u6574\u6027\u3002\u4f8b\u5982\uff0c\u5728\u67d0\u4e9b\u60c5\u6cc1\u4e0b\uff0cnull\u503c\u53ef\u4ee5\u7528\u4f86\u8868\u793a\u67d0\u4e9b\u5b57\u6bb5\u5728\u7279\u5b9a\u60c5\u6cc1\u4e0b\u4e0d\u9069\u7528\uff0c\u5f9e\u800c\u907f\u514d\u932f\u8aa4\u7684\u6578\u64da\u8f38\u5165\u3002<\/p>\n<h2 id=\"null%e5%80%bc%e7%9a%84%e6%8c%91%e6%88%b0-FyLfKWlBeW\">null\u503c\u7684\u6311\u6230<\/h2>\n<p>\u5118\u7ba1null\u503c\u5728\u6578\u64da\u5eab\u4e2d\u6709\u5176\u5fc5\u8981\u6027\uff0c\u4f46\u5b83\u5011\u4e5f\u5e36\u4f86\u4e86\u4e00\u4e9b\u6311\u6230\uff1a<\/p>\n<ul>\n<li><strong>\u67e5\u8a62\u8907\u96dc\u6027\uff1a<\/strong>\u4f7f\u7528null\u503c\u53ef\u80fd\u6703\u4f7f\u67e5\u8a62\u8b8a\u5f97\u66f4\u52a0\u8907\u96dc\uff0c\u7279\u5225\u662f\u5728\u9032\u884c\u805a\u5408\u8a08\u7b97\u6642\u3002<\/li>\n<li><strong>\u6578\u64da\u4e00\u81f4\u6027\uff1a<\/strong>\u5728\u6578\u64da\u5eab\u4e2d\uff0c\u904e\u591a\u7684null\u503c\u53ef\u80fd\u6703\u5c0e\u81f4\u6578\u64da\u7684\u4e00\u81f4\u6027\u554f\u984c\uff0c\u7279\u5225\u662f\u5728\u9032\u884c\u6578\u64da\u5206\u6790\u6642\u3002<\/li>\n<\/ul>\n<h2 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