{"id":200460,"date":"2025-05-06T12:49:58","date_gmt":"2025-05-06T04:49:58","guid":{"rendered":"https:\/\/server.hk\/cnblog\/200460\/"},"modified":"2025-05-06T12:49:58","modified_gmt":"2025-05-06T04:49:58","slug":"mysql-%e4%b8%ad%e5%8f%8c%e8%a1%a8-dual-%e6%9c%89%e4%bb%80%e4%b9%88%e7%94%a8%e9%80%94%e5%92%8c%e5%ba%94%e7%94%a8%e5%9c%ba%e6%99%af%ef%bc%9f","status":"publish","type":"post","link":"https:\/\/server.hk\/cnblog\/200460\/","title":{"rendered":"MySQL \u4e2d\u53cc\u8868 dual \u6709\u4ec0\u4e48\u7528\u9014\u548c\u5e94\u7528\u573a\u666f\uff1f"},"content":{"rendered":"

<\/b> <\/p>\n

MySQL \u4e2d\u53cc\u8868 dual \u6709\u4ec0\u4e48\u7528\u9014\u548c\u5e94\u7528\u573a\u666f\uff1f<\/h1>\n

\u6b22\u8fce\u5404\u4f4d\u5c0f\u4f19\u4f34\u6765\u5230\u4e3b\u673a\u5b9d\u8d1d\uff0c\u76f8\u805a\u4e8e\u6b64\u90fd\u662f\u7f18\u54c8\u54c8\u54c8\uff01\u4eca\u5929\u6211\u7ed9\u5927\u5bb6\u5e26\u6765\uff0c\u8fd9\u7bc7\u6587\u7ae0\u4e3b\u8981\u8bb2\u5230\u7b49\u7b49\u77e5\u8bc6\uff0c\u5982\u679c\u4f60\u5bf9\u76f8\u5173\u7684\u77e5\u8bc6\u975e\u5e38\u611f\u5174\u8da3\u6216\u8005\u6b63\u5728\u81ea\u5b66\uff0c\u90fd\u53ef\u4ee5\u5173\u6ce8\u6211\uff0c\u6211\u4f1a\u6301\u7eed\u66f4\u65b0\u76f8\u5173\u6587\u7ae0\uff01\u5f53\u7136\uff0c\u6709\u4ec0\u4e48\u5efa\u8bae\u4e5f\u6b22\u8fce\u5728\u8bc4\u8bba\u7559\u8a00\u63d0\u51fa\uff01\u4e00\u8d77\u5b66\u4e60\uff01<\/p>\n

<\/p>\n

mysql \u4e2d\u4f2a\u8868 dual \u7684\u4f5c\u7528\u548c\u5e94\u7528\u573a\u666f<\/strong><\/p>\n

\u5728 mysql \u4e2d\uff0cdual \u5e76\u4e0d\u662f\u4e00\u4e2a\u771f\u6b63\u610f\u4e49\u4e0a\u7684\u8868\uff0c\u800c\u662f\u4e00\u4e2a\u865a\u62df\u3001\u5185\u90e8\u521b\u5efa\u7684\u201c\u4f2a\u8868\u201d\uff0c\u5176\u4e2d\u53ea\u6709\u4e00\u884c\u6570\u636e\uff0c\u8be5\u884c\u5305\u542b\u4e00\u4e2a\u503c\u4e3a null \u7684\u5217\u3002\u867d\u7136 mysql \u4e2d\u6ca1\u6709\u6240\u8c13\u7684\u771f\u4f2a\u8868\uff0c\u4f46\u5b83\u53ef\u4ee5\u5728\u67d0\u4e9b\u7279\u5b9a\u573a\u666f\u4e0b\u63d0\u4f9b\u65b9\u4fbf\u3002<\/p>\n

\u67e5\u8be2\u533a\u522b<\/strong><\/p>\n

\u4f7f\u7528 dual \u7684\u4e3b\u8981\u539f\u56e0\u4e4b\u4e00\u662f\uff0c\u5b83\u4e3a\u4e0d\u9700\u8981\u5b9e\u9645\u6570\u636e\u7684\u67e5\u8be2\u63d0\u4f9b\u4e86\u4e00\u4e2a\u65b9\u4fbf\u7684\u65b9\u5f0f\u3002\u4f8b\u5982\uff1a<\/p>\n

select 6+6 from dual;<\/pre>\n
\u4e0e\u4e0d\u4f7f\u7528 dual \u7684\u67e5\u8be2\u76f8\u6bd4\uff1a<\/p>\n
select 6+6;<\/pre>\n
\u8fd9\u4e24\u79cd\u65b9\u5f0f\u4ea7\u751f\u7684\u7ed3\u679c\u76f8\u540c\uff0c\u90fd\u662f 12\u3002\u4f46\u5728\u67d0\u4e9b\u60c5\u51b5\u4e0b\uff0c\u4f7f\u7528 dual \u53ef\u80fd\u4f1a\u66f4\u6709\u4f18\u52bf\u3002\u4f8b\u5982\uff0c\u5728\u5b58\u50a8\u8fc7\u7a0b\u6216\u51fd\u6570\u4e2d\uff0c\u76f4\u63a5\u67e5\u8be2\u6570\u5b57\u65f6\uff0c\u53ef\u80fd\u4f1a\u51fa\u73b0\u8bed\u6cd5\u9519\u8bef\u3002\u800c\u4f7f\u7528 dual \u53ef\u4ee5\u907f\u514d\u8fd9\u4e2a\u95ee\u9898\uff1a<\/p>\n
SELECT COUNT(*) FROM table_name WHERE condition = (SELECT 6+6 FROM dual);<\/pre>\n
\u5e38\u89c1\u573a\u666f<\/strong><\/p>\n
mysql \u4e2d dual \u7684\u5e38\u89c1\u5e94\u7528\u573a\u666f\u5305\u62ec\uff1a<\/p>\n
    \n
  • \u4f5c\u4e3a\u5360\u4f4d\u7b26\uff1a<\/strong>\u5728\u6ca1\u6709\u5b9e\u9645\u6570\u636e\u7684\u67e5\u8be2\u4e2d\uff0cdual \u53ef\u7528\u4f5c\u5360\u4f4d\u7b26\u3002<\/li>\n
  • \u521d\u59cb\u5316\u53d8\u91cf\uff1a<\/strong>\u5728\u5b58\u50a8\u8fc7\u7a0b\u6216\u51fd\u6570\u4e2d\uff0cdual \u53ef\u7528\u4e8e\u521d\u59cb\u5316\u53d8\u91cf\u3002<\/li>\n
  • \u907f\u514d\u8bed\u6cd5\u9519\u8bef\uff1a<\/strong>\u5728\u9700\u8981\u4f7f\u7528\u6570\u5b57\u4f5c\u4e3a\u67e5\u8be2\u6761\u4ef6\u7684\u573a\u666f\u4e2d\uff0cdual \u53ef\u4ee5\u907f\u514d\u8bed\u6cd5\u9519\u8bef\u3002<\/li>\n<\/ul>\n
    \u503c\u5f97\u6ce8\u610f\u7684\u662f\uff0cdual \u4e3b\u8981\u7528\u4e8e oracle \u6570\u636e\u5e93\uff0c\u5728 mysql \u4e2d\u5e76\u975e\u5fc5\u9700\u3002\u5927\u591a\u6570\u60c5\u51b5\u4e0b\uff0c\u53ef\u4ee5\u76f4\u63a5\u67e5\u8be2\u6570\u5b57\u800c\u4e0d\u4f7f\u7528 dual\u3002<\/p>\n
    \u4ee5\u4e0a\u5c31\u662f\u300aMySQL \u4e2d\u53cc\u8868 dual \u6709\u4ec0\u4e48\u7528\u9014\u548c\u5e94\u7528\u573a\u666f\uff1f\u300b\u7684\u8be6\u7ec6\u5185\u5bb9\uff0c\u66f4\u591a\u5173\u4e8e\u7684\u8d44\u6599\u8bf7\u5173\u6ce8\u4e3b\u673a\u5b9d\u8d1d\u516c\u4f17\u53f7\uff01<\/p>\n
     \u7248\u672c\u58f0\u660e \u672c\u6587\u8f6c\u8f7d\u4e8e\uff1aphp \u5982\u6709\u4fb5\u72af\uff0c\u8bf7\u8054\u7cfb \u5220\u9664 <\/p>\n
    \n
    \n <\/dt>\n<\/dl>\n","protected":false},"excerpt":{"rendered":"
    MySQL \u4e2d\u53cc\u8868 dual \u6709...<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[101],"tags":[],"class_list":["post-200460","post","type-post","status-publish","format-standard","hentry","category-database"],"_links":{"self":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/200460","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"}],"author":[{"embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/comments?post=200460"}],"version-history":[{"count":0,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/200460\/revisions"}],"wp:attachment":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/media?parent=200460"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/categories?post=200460"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/tags?post=200460"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}