{"id":78060,"date":"2024-10-16T11:28:49","date_gmt":"2024-10-16T03:28:49","guid":{"rendered":"https:\/\/server.hk\/cnblog\/78060\/"},"modified":"2024-10-16T11:28:49","modified_gmt":"2024-10-16T03:28:49","slug":"mysql%e6%8c%87%e4%bb%a4-count%ef%bc%88%e8%a8%88%e6%95%b8%ef%bc%89","status":"publish","type":"post","link":"https:\/\/server.hk\/cnblog\/78060\/","title":{"rendered":"MySQL\u6307\u4ee4: COUNT()\uff08\u8a08\u6578\uff09"},"content":{"rendered":"<h1 id=\"mysql%e6%8c%87%e4%bb%a4-count%ef%bc%88%e8%a8%88%e6%95%b8%ef%bc%89-pkKBFMbXGn\">MySQL\u6307\u4ee4: COUNT()\uff08\u8a08\u6578\uff09<\/h1>\n<p>\u5728\u8cc7\u6599\u5eab\u7ba1\u7406\u7cfb\u7d71\u4e2d\uff0cMySQL\u662f\u4e00\u500b\u5ee3\u6cdb\u4f7f\u7528\u7684\u958b\u6e90\u8cc7\u6599\u5eab\uff0c\u7279\u5225\u662f\u5728\u7db2\u9801\u61c9\u7528\u7a0b\u5f0f\u4e2d\u3002MySQL\u63d0\u4f9b\u4e86\u591a\u7a2e\u529f\u80fd\u5f37\u5927\u7684\u6307\u4ee4\uff0c\u5176\u4e2d\u4e4b\u4e00\u5c31\u662fCOUNT()\u51fd\u6578\u3002\u9019\u500b\u51fd\u6578\u4e3b\u8981\u7528\u65bc\u8a08\u7b97\u8cc7\u6599\u8868\u4e2d\u7b26\u5408\u7279\u5b9a\u689d\u4ef6\u7684\u8a18\u9304\u6578\u91cf\u3002\u672c\u6587\u5c07\u6df1\u5165\u63a2\u8a0eCOUNT()\u51fd\u6578\u7684\u7528\u6cd5\u53ca\u5176\u5728\u5be6\u969b\u61c9\u7528\u4e2d\u7684\u91cd\u8981\u6027\u3002<\/p>\n<h2 id=\"count%e5%87%bd%e6%95%b8%e7%9a%84%e5%9f%ba%e6%9c%ac%e8%aa%9e%e6%b3%95-pkKBFMbXGn\">COUNT()\u51fd\u6578\u7684\u57fa\u672c\u8a9e\u6cd5<\/h2>\n<p>COUNT()\u51fd\u6578\u7684\u57fa\u672c\u8a9e\u6cd5\u5982\u4e0b\uff1a<\/p>\n<pre><code>SELECT COUNT(column_name) FROM table_name WHERE condition;<\/code><\/pre>\n<p>\u5728\u9019\u88e1\uff0c<code>column_name<\/code>\u662f\u4f60\u60f3\u8981\u8a08\u6578\u7684\u6b04\u4f4d\u540d\u7a31\uff0c<code>table_name<\/code>\u662f\u8cc7\u6599\u8868\u7684\u540d\u7a31\uff0c\u800c<code>condition<\/code>\u5247\u662f\u53ef\u9078\u7684\u689d\u4ef6\uff0c\u7528\u65bc\u904e\u6ffe\u8a08\u6578\u7684\u8a18\u9304\u3002<\/p>\n<h2 id=\"count%e5%87%bd%e6%95%b8%e7%9a%84%e4%bd%bf%e7%94%a8%e7%af%84%e4%be%8b-pkKBFMbXGn\">COUNT()\u51fd\u6578\u7684\u4f7f\u7528\u7bc4\u4f8b<\/h2>\n<p>\u4ee5\u4e0b\u662f\u4e00\u4e9b\u4f7f\u7528COUNT()\u51fd\u6578\u7684\u5be6\u969b\u7bc4\u4f8b\uff1a<\/p>\n<h3 id=\"1-%e8%a8%88%e7%ae%97%e6%89%80%e6%9c%89%e8%a8%98%e9%8c%84%e7%9a%84%e6%95%b8%e9%87%8f-pkKBFMbXGn\">1. \u8a08\u7b97\u6240\u6709\u8a18\u9304\u7684\u6578\u91cf<\/h3>\n<p>\u5982\u679c\u4f60\u60f3\u8981\u8a08\u7b97\u67d0\u500b\u8cc7\u6599\u8868\u4e2d\u6240\u6709\u8a18\u9304\u7684\u6578\u91cf\uff0c\u53ef\u4ee5\u4f7f\u7528\u4ee5\u4e0b\u6307\u4ee4\uff1a<\/p>\n<pre><code>SELECT COUNT(*) FROM employees;<\/code><\/pre>\n<p>\u9019\u689d\u6307\u4ee4\u5c07\u8fd4\u56de<code>employees<\/code>\u8cc7\u6599\u8868\u4e2d\u6240\u6709\u8a18\u9304\u7684\u7e3d\u6578\u3002<\/p>\n<h3 id=\"2-%e8%a8%88%e7%ae%97%e7%89%b9%e5%ae%9a%e6%a2%9d%e4%bb%b6%e4%b8%8b%e7%9a%84%e8%a8%98%e9%8c%84%e6%95%b8%e9%87%8f-pkKBFMbXGn\">2. \u8a08\u7b97\u7279\u5b9a\u689d\u4ef6\u4e0b\u7684\u8a18\u9304\u6578\u91cf<\/h3>\n<p>\u5047\u8a2d\u4f60\u60f3\u8981\u8a08\u7b97\u6240\u6709\u5728\u67d0\u500b\u90e8\u9580\u5de5\u4f5c\u7684\u54e1\u5de5\u6578\u91cf\uff0c\u53ef\u4ee5\u9019\u6a23\u5beb\uff1a<\/p>\n<pre><code>SELECT COUNT(*) FROM employees WHERE department = 'Sales';<\/code><\/pre>\n<p>\u9019\u689d\u6307\u4ee4\u5c07\u8fd4\u56de<code>Sales<\/code>\u90e8\u9580\u4e2d\u54e1\u5de5\u7684\u6578\u91cf\u3002<\/p>\n<h3 id=\"3-%e8%a8%88%e7%ae%97%e4%b8%8d%e9%87%8d%e8%a4%87%e7%9a%84%e8%a8%98%e9%8c%84%e6%95%b8%e9%87%8f-pkKBFMbXGn\">3. \u8a08\u7b97\u4e0d\u91cd\u8907\u7684\u8a18\u9304\u6578\u91cf<\/h3>\n<p>\u5982\u679c\u4f60\u9700\u8981\u8a08\u7b97\u67d0\u500b\u6b04\u4f4d\u4e2d\u4e0d\u91cd\u8907\u7684\u503c\u7684\u6578\u91cf\uff0c\u53ef\u4ee5\u4f7f\u7528<code>COUNT(DISTINCT column_name)<\/code>\uff1a<\/p>\n<pre><code>SELECT COUNT(DISTINCT department) FROM employees;<\/code><\/pre>\n<p>\u9019\u689d\u6307\u4ee4\u5c07\u8fd4\u56de<code>employees<\/code>\u8cc7\u6599\u8868\u4e2d\u4e0d\u540c\u90e8\u9580\u7684\u6578\u91cf\u3002<\/p>\n<h2 id=\"count%e5%87%bd%e6%95%b8%e7%9a%84%e6%87%89%e7%94%a8%e5%a0%b4%e6%99%af-pkKBFMbXGn\">COUNT()\u51fd\u6578\u7684\u61c9\u7528\u5834\u666f<\/h2>\n<p>COUNT()\u51fd\u6578\u5728\u8a31\u591a\u5834\u666f\u4e2d\u90fd\u975e\u5e38\u6709\u7528\uff0c\u4ee5\u4e0b\u662f\u4e00\u4e9b\u5e38\u898b\u7684\u61c9\u7528\uff1a<\/p>\n<ul>\n<li><strong>\u5831\u544a\u751f\u6210\uff1a<\/strong>\u5728\u751f\u6210\u696d\u52d9\u5831\u544a\u6642\uff0cCOUNT()\u51fd\u6578\u53ef\u4ee5\u5e6b\u52a9\u5feb\u901f\u7372\u53d6\u5404\u985e\u6578\u64da\u7684\u7d71\u8a08\u4fe1\u606f\u3002<\/li>\n<li><strong>\u6578\u64da\u5206\u6790\uff1a<\/strong>\u5728\u9032\u884c\u6578\u64da\u5206\u6790\u6642\uff0cCOUNT()\u51fd\u6578\u53ef\u4ee5\u7528\u4f86\u8a55\u4f30\u7279\u5b9a\u689d\u4ef6\u4e0b\u7684\u6578\u64da\u5206\u4f48\u3002<\/li>\n<li><strong>\u7528\u6236\u7ba1\u7406\uff1a<\/strong>\u5728\u7528\u6236\u7ba1\u7406\u7cfb\u7d71\u4e2d\uff0cCOUNT()\u51fd\u6578\u53ef\u4ee5\u7528\u4f86\u8a08\u7b97\u6d3b\u8e8d\u7528\u6236\u6216\u8a3b\u518a\u7528\u6236\u7684\u6578\u91cf\u3002<\/li>\n<\/ul>\n<h2 id=\"%e6%b3%a8%e6%84%8f%e4%ba%8b%e9%a0%85-pkKBFMbXGn\">\u6ce8\u610f\u4e8b\u9805<\/h2>\n<p>\u5728\u4f7f\u7528COUNT()\u51fd\u6578\u6642\uff0c\u6709\u5e7e\u9ede\u9700\u8981\u6ce8\u610f\uff1a<\/p>\n<ul>\n<li>\u4f7f\u7528<code>COUNT(*)<\/code>\u6703\u8a08\u7b97\u6240\u6709\u8a18\u9304\uff0c\u5305\u62ecNULL\u503c\uff0c\u800c<code>COUNT(column_name)<\/code>\u5247\u4e0d\u6703\u8a08\u7b97NULL\u503c\u3002<\/li>\n<li>\u5728\u5927\u578b\u8cc7\u6599\u8868\u4e2d\uff0cCOUNT()\u51fd\u6578\u7684\u57f7\u884c\u53ef\u80fd\u6703\u5f71\u97ff\u6027\u80fd\uff0c\u56e0\u6b64\u5728\u8a2d\u8a08\u67e5\u8a62\u6642\u61c9\u8b39\u614e\u8003\u616e\u3002<\/li>\n<\/ul>\n<h2 id=\"%e7%b8%bd%e7%b5%90-pkKBFMbXGn\">\u7e3d\u7d50<\/h2>\n<p>COUNT()\u51fd\u6578\u662fMySQL\u4e2d\u4e00\u500b\u975e\u5e38\u5be6\u7528\u7684\u5de5\u5177\uff0c\u80fd\u5920\u5e6b\u52a9\u7528\u6236\u5feb\u901f\u7372\u53d6\u8cc7\u6599\u8868\u4e2d\u7684\u8a18\u9304\u6578\u91cf\u3002\u7121\u8ad6\u662f\u5728\u5831\u544a\u751f\u6210\u3001\u6578\u64da\u5206\u6790\u9084\u662f\u7528\u6236\u7ba1\u7406\u4e2d\uff0cCOUNT()\u51fd\u6578\u90fd\u80fd\u767c\u63ee\u91cd\u8981\u4f5c\u7528\u3002\u5c0d\u65bc\u9700\u8981\u9ad8\u6548\u6578\u64da\u8655\u7406\u7684\u61c9\u7528\uff0c\u9078\u64c7\u5408\u9069\u7684<code>VPS<\/code>\u89e3\u6c7a\u65b9\u6848\uff0c\u5982<a href=\"https:\/\/server.hk\">\u9999\u6e2fVPS<\/a>\uff0c\u53ef\u4ee5\u9032\u4e00\u6b65\u63d0\u5347\u8cc7\u6599\u5eab\u7684\u6027\u80fd\u548c\u7a69\u5b9a\u6027\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u4e86\u89e3MySQL\u4e2d\u7684COUNT()\u6307\u4ee4\uff0c\u8f15\u9b06\u8a08\u7b97\u8cc7\u6599\u8868\u4e2d\u7684\u8a18\u9304\u6578\uff0c\u63d0\u5347\u6578\u64da\u5206\u6790\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-78060","post","type-post","status-publish","format-standard","hentry","category-database"],"_links":{"self":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/78060","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=78060"}],"version-history":[{"count":1,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/78060\/revisions"}],"predecessor-version":[{"id":78061,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/78060\/revisions\/78061"}],"wp:attachment":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/media?parent=78060"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/categories?post=78060"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/tags?post=78060"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}