{"id":79328,"date":"2024-10-16T12:55:17","date_gmt":"2024-10-16T04:55:17","guid":{"rendered":"https:\/\/server.hk\/cnblog\/79328\/"},"modified":"2024-10-16T12:55:17","modified_gmt":"2024-10-16T04:55:17","slug":"mysql%e6%8c%87%e4%bb%a4-null%ef%bc%88%e7%a9%ba%e5%80%bc%ef%bc%89","status":"publish","type":"post","link":"https:\/\/server.hk\/cnblog\/79328\/","title":{"rendered":"MySQL\u6307\u4ee4: NULL\uff08\u7a7a\u503c\uff09"},"content":{"rendered":"<h1 id=\"mysql%e6%8c%87%e4%bb%a4-null%ef%bc%88%e7%a9%ba%e5%80%bc%ef%bc%89-EtGUpVyHCh\">MySQL\u6307\u4ee4: NULL\uff08\u7a7a\u503c\uff09<\/h1>\n<p>\u5728\u8cc7\u6599\u5eab\u7ba1\u7406\u7cfb\u7d71\u4e2d\uff0cNULL\uff08\u7a7a\u503c\uff09\u662f\u4e00\u500b\u91cd\u8981\u7684\u6982\u5ff5\uff0c\u7279\u5225\u662f\u5728\u4f7f\u7528MySQL\u6642\u3002NULL\u4e26\u4e0d\u7b49\u540c\u65bc\u96f6\u6216\u7a7a\u5b57\u4e32\uff0c\u800c\u662f\u8868\u793a\u4e00\u500b\u672a\u77e5\u6216\u7f3a\u5931\u7684\u503c\u3002\u7406\u89e3NULL\u7684\u7279\u6027\u53ca\u5176\u5728MySQL\u4e2d\u7684\u4f7f\u7528\uff0c\u5c0d\u65bc\u8a2d\u8a08\u548c\u67e5\u8a62\u8cc7\u6599\u5eab\u81f3\u95dc\u91cd\u8981\u3002<\/p>\n<h2 id=\"null%e7%9a%84%e5%ae%9a%e7%be%a9-EtGUpVyHCh\">NULL\u7684\u5b9a\u7fa9<\/h2>\n<p>\u5728MySQL\u4e2d\uff0cNULL\u662f\u4e00\u500b\u7279\u6b8a\u7684\u6a19\u8a18\uff0c\u7528\u4f86\u8868\u793a\u8cc7\u6599\u7684\u7f3a\u5931\u6216\u672a\u77e5\u3002\u7576\u4e00\u500b\u6b04\u4f4d\u7684\u503c\u70baNULL\u6642\uff0c\u9019\u610f\u5473\u8457\u8a72\u6b04\u4f4d\u6c92\u6709\u4efb\u4f55\u6709\u6548\u7684\u6578\u64da\u3002\u9019\u8207\u6578\u5b570\u6216\u7a7a\u5b57\u4e32\uff08&#8221;&#8221;\uff09\u662f\u4e0d\u540c\u7684\uff0c\u56e0\u70ba\u9019\u4e9b\u503c\u662f\u6709\u6548\u7684\u6578\u64da\u3002<\/p>\n<h2 id=\"null%e7%9a%84%e7%89%b9%e6%80%a7-EtGUpVyHCh\">NULL\u7684\u7279\u6027<\/h2>\n<ul>\n<li><strong>\u4e0d\u7b49\u65bc\u4efb\u4f55\u503c\uff1a<\/strong>\u5728MySQL\u4e2d\uff0cNULL\u4e0d\u7b49\u65bc\u4efb\u4f55\u503c\uff0c\u5305\u62ec\u5b83\u81ea\u5df1\u3002\u9019\u610f\u5473\u8457\u5982\u679c\u4f60\u57f7\u884c\u4e00\u500b\u6bd4\u8f03\uff0c\u4f8b\u5982 <code>NULL = NULL<\/code>\uff0c\u7d50\u679c\u5c07\u662fFALSE\u3002<\/li>\n<li><strong>\u4f7f\u7528IS NULL\u548cIS NOT NULL\uff1a<\/strong>\u8981\u6aa2\u67e5\u4e00\u500b\u6b04\u4f4d\u662f\u5426\u70baNULL\uff0c\u61c9\u4f7f\u7528 <code>IS NULL<\/code> \u6216 <code>IS NOT NULL<\/code>\u3002\u4f8b\u5982\uff1a<\/li>\n<\/ul>\n<pre><code>SELECT * FROM users WHERE email IS NULL;<\/code><\/pre>\n<p>\u9019\u689d\u67e5\u8a62\u5c07\u8fd4\u56de\u6240\u6709\u96fb\u5b50\u90f5\u4ef6\u6b04\u4f4d\u70baNULL\u7684\u7528\u6236\u3002<\/p>\n<h2 id=\"null%e5%9c%a8%e6%9f%a5%e8%a9%a2%e4%b8%ad%e7%9a%84%e6%87%89%e7%94%a8-EtGUpVyHCh\">NULL\u5728\u67e5\u8a62\u4e2d\u7684\u61c9\u7528<\/h2>\n<p>\u5728\u9032\u884c\u67e5\u8a62\u6642\uff0cNULL\u7684\u8655\u7406\u65b9\u5f0f\u53ef\u80fd\u6703\u5f71\u97ff\u7d50\u679c\u3002\u4f8b\u5982\uff0c\u7576\u4f7f\u7528\u805a\u5408\u51fd\u6578\u6642\uff0cNULL\u503c\u6703\u88ab\u5ffd\u7565\u3002\u4ee5\u4e0b\u662f\u4e00\u500b\u793a\u4f8b\uff1a<\/p>\n<pre><code>SELECT COUNT(*) FROM orders WHERE discount IS NULL;<\/code><\/pre>\n<p>\u9019\u689d\u67e5\u8a62\u5c07\u8a08\u7b97\u6240\u6709\u6298\u6263\u70baNULL\u7684\u8a02\u55ae\u6578\u91cf\u3002<\/p>\n<h2 id=\"%e6%8f%92%e5%85%a5null%e5%80%bc-EtGUpVyHCh\">\u63d2\u5165NULL\u503c<\/h2>\n<p>\u5728\u63d2\u5165\u6578\u64da\u6642\uff0c\u53ef\u4ee5\u660e\u78ba\u5730\u5c07NULL\u503c\u63d2\u5165\u5230\u67d0\u500b\u6b04\u4f4d\u4e2d\u3002\u4ee5\u4e0b\u662f\u4e00\u500b\u793a\u4f8b\uff1a<\/p>\n<pre><code>INSERT INTO users (username, email) VALUES ('john_doe', NULL);<\/code><\/pre>\n<p>\u5728\u9019\u500b\u4f8b\u5b50\u4e2d\uff0c\u4f7f\u7528\u8005\u7684\u96fb\u5b50\u90f5\u4ef6\u6b04\u4f4d\u88ab\u8a2d\u7f6e\u70baNULL\u3002<\/p>\n<h2 id=\"%e6%9b%b4%e6%96%b0null%e5%80%bc-EtGUpVyHCh\">\u66f4\u65b0NULL\u503c<\/h2>\n<p>\u5982\u679c\u9700\u8981\u5c07\u67d0\u500b\u6b04\u4f4d\u7684\u503c\u66f4\u65b0\u70baNULL\uff0c\u53ef\u4ee5\u4f7f\u7528\u4ee5\u4e0b\u8a9e\u6cd5\uff1a<\/p>\n<pre><code>UPDATE users SET email = NULL WHERE username = 'john_doe';<\/code><\/pre>\n<p>\u9019\u689d\u67e5\u8a62\u5c07\u628a\u7528\u6236\u540d\u70ba&#8217;john_doe&#8217;\u7684\u7528\u6236\u7684\u96fb\u5b50\u90f5\u4ef6\u6b04\u4f4d\u8a2d\u7f6e\u70baNULL\u3002<\/p>\n<h2 id=\"null%e7%9a%84%e6%b3%a8%e6%84%8f%e4%ba%8b%e9%a0%85-EtGUpVyHCh\">NULL\u7684\u6ce8\u610f\u4e8b\u9805<\/h2>\n<p>\u5728\u4f7f\u7528NULL\u6642\uff0c\u6709\u5e7e\u500b\u6ce8\u610f\u4e8b\u9805\u9700\u8981\u8003\u616e\uff1a<\/p>\n<ul>\n<li>\u5728\u9032\u884c\u6578\u64da\u6bd4\u8f03\u6642\uff0c\u5fc5\u9808\u4f7f\u7528 <code>IS NULL<\/code> \u6216 <code>IS NOT NULL<\/code>\u3002<\/li>\n<li>NULL\u503c\u5728\u805a\u5408\u51fd\u6578\u4e2d\u6703\u88ab\u5ffd\u7565\uff0c\u9019\u53ef\u80fd\u6703\u5f71\u97ff\u8a08\u7b97\u7d50\u679c\u3002<\/li>\n<li>\u5728\u8a2d\u8a08\u8cc7\u6599\u5eab\u6642\uff0c\u61c9\u8003\u616e\u54ea\u4e9b\u6b04\u4f4d\u53ef\u4ee5\u63a5\u53d7NULL\u503c\uff0c\u54ea\u4e9b\u6b04\u4f4d\u5fc5\u9808\u6709\u6709\u6548\u6578\u64da\u3002<\/li>\n<\/ul>\n<h2 id=\"%e7%b5%90%e8%ab%96-EtGUpVyHCh\">\u7d50\u8ad6<\/h2>\n<p>NULL\u5728MySQL\u4e2d\u662f\u4e00\u500b\u91cd\u8981\u7684\u6982\u5ff5\uff0c\u6b63\u78ba\u7406\u89e3\u548c\u4f7f\u7528NULL\u53ef\u4ee5\u5e6b\u52a9\u958b\u767c\u8005\u66f4\u6709\u6548\u5730\u8a2d\u8a08\u548c\u67e5\u8a62\u8cc7\u6599\u5eab\u3002\u7121\u8ad6\u662f\u5728\u63d2\u5165\u3001\u66f4\u65b0\u9084\u662f\u67e5\u8a62\u6578\u64da\u6642\uff0c\u5c0dNULL\u7684\u6b63\u78ba\u8655\u7406\u90fd\u662f\u81f3\u95dc\u91cd\u8981\u7684\u3002<\/p>\n<p>\u5982\u9700\u4e86\u89e3\u66f4\u591a\u95dc\u65bc\u8cc7\u6599\u5eab\u7ba1\u7406\u548c\u76f8\u95dc\u6280\u8853\u7684\u8cc7\u8a0a\uff0c\u60a8\u53ef\u4ee5\u8a2a\u554f\u6211\u5011\u7684\u7db2\u7ad9 <a href=\"https:\/\/server.hk\">Server.HK<\/a>\uff0c\u63a2\u7d22\u6211\u5011\u7684 <a href=\"https:\/\/server.hk\">VPS<\/a> \u89e3\u6c7a\u65b9\u6848\u53ca\u5176\u4ed6\u670d\u52d9\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u4e86\u89e3MySQL\u4e2d\u7684NULL\uff08\u7a7a\u503c\uff09\u6307\u4ee4\uff0c\u638c\u63e1\u5982\u4f55\u5904\u7406\u548c\u67e5\u8be2\u7a7a\u503c\u6570\u636e\uff0c\u63d0\u9ad8\u6570\u636e\u5e93\u7ba1\u7406\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-79328","post","type-post","status-publish","format-standard","hentry","category-database"],"_links":{"self":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/79328","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=79328"}],"version-history":[{"count":1,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/79328\/revisions"}],"predecessor-version":[{"id":79329,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/79328\/revisions\/79329"}],"wp:attachment":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/media?parent=79328"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/categories?post=79328"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/tags?post=79328"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}