{"id":186266,"date":"2024-11-11T04:29:21","date_gmt":"2024-11-10T20:29:21","guid":{"rendered":"https:\/\/server.hk\/cnblog\/186266\/"},"modified":"2024-11-11T04:29:22","modified_gmt":"2024-11-10T20:29:22","slug":"%e6%95%b8%e6%93%9a%e5%ba%ab%e6%9f%a5%e8%a9%a2%e8%aa%9e%e5%8f%a5count%e7%9a%84%e4%bd%bf%e7%94%a8%e6%96%b9%e6%b3%95%e5%92%8c%e6%b3%a8%e6%84%8f%e4%ba%8b%e9%a0%85-%e6%95%b8%e6%93%9a%e5%ba%ab%e6%9f%a5","status":"publish","type":"post","link":"https:\/\/server.hk\/cnblog\/186266\/","title":{"rendered":"\u6578\u64da\u5eab\u67e5\u8a62\u8a9e\u53e5count\u7684\u4f7f\u7528\u65b9\u6cd5\u548c\u6ce8\u610f\u4e8b\u9805 (\u6578\u64da\u5eab\u67e5\u8a62\u8a9e\u53e5count)"},"content":{"rendered":"<h1 id=\"%e6%95%b8%e6%93%9a%e5%ba%ab%e6%9f%a5%e8%a9%a2%e8%aa%9e%e5%8f%a5count%e7%9a%84%e4%bd%bf%e7%94%a8%e6%96%b9%e6%b3%95%e5%92%8c%e6%b3%a8%e6%84%8f%e4%ba%8b%e9%a0%85-xDIvSeHfMR\">\u6578\u64da\u5eab\u67e5\u8a62\u8a9e\u53e5count\u7684\u4f7f\u7528\u65b9\u6cd5\u548c\u6ce8\u610f\u4e8b\u9805<\/h1>\n<p>\u5728\u6578\u64da\u5eab\u7ba1\u7406\u4e2d\uff0c\u67e5\u8a62\u8a9e\u53e5\u662f\u7528\u4f86\u5f9e\u6578\u64da\u5eab\u4e2d\u6aa2\u7d22\u6578\u64da\u7684\u57fa\u672c\u5de5\u5177\u3002\u5176\u4e2d\uff0c<code>COUNT<\/code> \u51fd\u6578\u662f\u4e00\u500b\u975e\u5e38\u91cd\u8981\u7684\u805a\u5408\u51fd\u6578\uff0c\u7528\u65bc\u8a08\u7b97\u7b26\u5408\u7279\u5b9a\u689d\u4ef6\u7684\u884c\u6578\u3002\u672c\u6587\u5c07\u6df1\u5165\u63a2\u8a0e <code>COUNT<\/code> \u51fd\u6578\u7684\u4f7f\u7528\u65b9\u6cd5\u53ca\u5176\u6ce8\u610f\u4e8b\u9805\uff0c\u5e6b\u52a9\u8b80\u8005\u66f4\u597d\u5730\u7406\u89e3\u548c\u61c9\u7528\u9019\u4e00\u529f\u80fd\u3002<\/p>\n<h2 id=\"count%e5%87%bd%e6%95%b8%e7%9a%84%e5%9f%ba%e6%9c%ac%e7%94%a8%e6%b3%95-xDIvSeHfMR\">COUNT\u51fd\u6578\u7684\u57fa\u672c\u7528\u6cd5<\/h2>\n<p><code>COUNT<\/code> \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\u8981\u8a08\u7b97\u7684\u5217\u540d\uff0c<code>table_name<\/code> \u662f\u6578\u64da\u8868\u7684\u540d\u7a31\uff0c\u800c <code>condition<\/code> \u5247\u662f\u53ef\u9078\u7684\u689d\u4ef6\uff0c\u7528\u65bc\u904e\u6ffe\u6578\u64da\u3002<\/p>\n<h3 id=\"%e7%a4%ba%e4%be%8b-xDIvSeHfMR\">\u793a\u4f8b<\/h3>\n<p>\u5047\u8a2d\u6211\u5011\u6709\u4e00\u500b\u540d\u70ba <code>employees<\/code> \u7684\u8868\uff0c\u5305\u542b\u4ee5\u4e0b\u6578\u64da\uff1a<\/p>\n<pre><code>\n+----+----------+--------+\n| id | name     | salary |\n+----+----------+--------+\n| 1  | Alice    | 5000   |\n| 2  | Bob      | 6000   |\n| 3  | Charlie  | 7000   |\n| 4  | David    | 8000   |\n| 5  | Eve      | 5000   |\n+----+----------+--------+\n<\/code><\/pre>\n<p>\u5982\u679c\u6211\u5011\u60f3\u8a08\u7b97 <code>employees<\/code> \u8868\u4e2d\u6240\u6709\u54e1\u5de5\u7684\u6578\u91cf\uff0c\u53ef\u4ee5\u4f7f\u7528\u4ee5\u4e0b\u67e5\u8a62\uff1a<\/p>\n<pre><code>SELECT COUNT(*) FROM employees;<\/code><\/pre>\n<p>\u9019\u5c07\u8fd4\u56de <code>5<\/code>\uff0c\u56e0\u70ba\u8868\u4e2d\u6709\u4e94\u540d\u54e1\u5de5\u3002<\/p>\n<h2 id=\"count%e5%87%bd%e6%95%b8%e7%9a%84%e4%b8%8d%e5%90%8c%e7%94%a8%e6%b3%95-xDIvSeHfMR\">COUNT\u51fd\u6578\u7684\u4e0d\u540c\u7528\u6cd5<\/h2>\n<p>\u9664\u4e86\u8a08\u7b97\u6240\u6709\u884c\u6578\u5916\uff0c<code>COUNT<\/code> \u51fd\u6578\u9084\u53ef\u4ee5\u7528\u4f86\u8a08\u7b97\u7279\u5b9a\u689d\u4ef6\u4e0b\u7684\u884c\u6578\u3002\u4f8b\u5982\uff0c\u5982\u679c\u6211\u5011\u60f3\u8a08\u7b97\u85aa\u6c34\u9ad8\u65bc <code>6000<\/code> \u7684\u54e1\u5de5\u6578\u91cf\uff0c\u53ef\u4ee5\u9019\u6a23\u5beb\uff1a<\/p>\n<pre><code>SELECT COUNT(*) FROM employees WHERE salary &gt; 6000;<\/code><\/pre>\n<p>\u9019\u5c07\u8fd4\u56de <code>3<\/code>\uff0c\u56e0\u70ba\u6709\u4e09\u540d\u54e1\u5de5\u7684\u85aa\u6c34\u9ad8\u65bc <code>6000<\/code>\u3002<\/p>\n<h3 id=\"%e4%bd%bf%e7%94%a8distinct%e9%97%9c%e9%8d%b5%e5%ad%97-xDIvSeHfMR\">\u4f7f\u7528DISTINCT\u95dc\u9375\u5b57<\/h3>\n<p>\u5728\u67d0\u4e9b\u60c5\u6cc1\u4e0b\uff0c\u6211\u5011\u53ef\u80fd\u53ea\u60f3\u8a08\u7b97\u552f\u4e00\u503c\u7684\u6578\u91cf\u3002\u9019\u6642\u53ef\u4ee5\u4f7f\u7528 <code>DISTINCT<\/code> \u95dc\u9375\u5b57\u3002\u4f8b\u5982\uff0c\u5982\u679c\u6211\u5011\u60f3\u8a08\u7b97\u4e0d\u540c\u85aa\u6c34\u7684\u6578\u91cf\uff0c\u53ef\u4ee5\u9019\u6a23\u5beb\uff1a<\/p>\n<pre><code>SELECT COUNT(DISTINCT salary) FROM employees;<\/code><\/pre>\n<p>\u9019\u5c07\u8fd4\u56de <code>4<\/code>\uff0c\u56e0\u70ba\u8868\u4e2d\u6709\u56db\u500b\u4e0d\u540c\u7684\u85aa\u6c34\u503c\u3002<\/p>\n<h2 id=\"%e6%b3%a8%e6%84%8f%e4%ba%8b%e9%a0%85-xDIvSeHfMR\">\u6ce8\u610f\u4e8b\u9805<\/h2>\n<ul>\n<li><strong>NULL\u503c\u7684\u8655\u7406\uff1a<\/strong>\u4f7f\u7528 <code>COUNT(column_name)<\/code> \u6642\uff0c<code>NULL<\/code> \u503c\u4e0d\u6703\u88ab\u8a08\u7b97\u5728\u5167\u3002\u5982\u679c\u9700\u8981\u8a08\u7b97\u6240\u6709\u884c\u6578\uff0c\u61c9\u4f7f\u7528 <code>COUNT(*)<\/code>\u3002<\/li>\n<li><strong>\u6027\u80fd\u8003\u91cf\uff1a<\/strong>\u5728\u5927\u578b\u6578\u64da\u96c6\u4e0a\u4f7f\u7528 <code>COUNT<\/code> \u51fd\u6578\u6642\uff0c\u53ef\u80fd\u6703\u5f71\u97ff\u67e5\u8a62\u6027\u80fd\u3002\u5efa\u8b70\u5728\u5fc5\u8981\u6642\u4f7f\u7528\u7d22\u5f15\u4f86\u63d0\u9ad8\u67e5\u8a62\u6548\u7387\u3002<\/li>\n<li><strong>\u805a\u5408\u51fd\u6578\u7684\u642d\u914d\uff1a<\/strong>\u53ef\u4ee5\u8207\u5176\u4ed6\u805a\u5408\u51fd\u6578\u4e00\u8d77\u4f7f\u7528\uff0c\u4f8b\u5982 <code>GROUP BY<\/code>\uff0c\u4ee5\u7372\u5f97\u66f4\u8a73\u7d30\u7684\u7d71\u8a08\u4fe1\u606f\u3002<\/li>\n<\/ul>\n<h2 id=\"%e7%b8%bd%e7%b5%90-xDIvSeHfMR\">\u7e3d\u7d50<\/h2>\n<p>\u5728\u6578\u64da\u5eab\u67e5\u8a62\u4e2d\uff0c<code>COUNT<\/code> \u51fd\u6578\u662f\u4e00\u500b\u5f37\u5927\u7684\u5de5\u5177\uff0c\u80fd\u5920\u5e6b\u52a9\u7528\u6236\u5feb\u901f\u7372\u53d6\u6578\u64da\u7d71\u8a08\u4fe1\u606f\u3002\u7121\u8ad6\u662f\u8a08\u7b97\u6240\u6709\u884c\u6578\u3001\u7279\u5b9a\u689d\u4ef6\u4e0b\u7684\u884c\u6578\uff0c\u9084\u662f\u552f\u4e00\u503c\u7684\u6578\u91cf\uff0c<code>COUNT<\/code> \u51fd\u6578\u90fd\u80fd\u63d0\u4f9b\u6709\u6548\u7684\u89e3\u6c7a\u65b9\u6848\u3002\u7136\u800c\uff0c\u5728\u4f7f\u7528\u6642\u9700\u6ce8\u610f\u8655\u7406 <code>NULL<\/code> \u503c\u3001\u67e5\u8a62\u6027\u80fd\u53ca\u805a\u5408\u51fd\u6578\u7684\u642d\u914d\u7b49\u554f\u984c\u3002\u5c0d\u65bc\u9700\u8981\u9ad8\u6548\u6578\u64da\u8655\u7406\u7684\u7528\u6236\uff0c\u9078\u64c7\u5408\u9069\u7684 <a href=\"https:\/\/server.hk\">VPS<\/a> \u89e3\u6c7a\u65b9\u6848\u5c07\u662f\u660e\u667a\u4e4b\u8209\uff0c\u56e0\u70ba\u5b83\u80fd\u63d0\u4f9b\u7a69\u5b9a\u7684\u6027\u80fd\u548c\u9748\u6d3b\u7684\u8cc7\u6e90\u914d\u7f6e\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u4e86\u89e3\u6578\u64da\u5eab\u67e5\u8a62\u8a9e\u53e5\u4e2dcount\u7684\u4f7f\u7528\u65b9\u6cd5\u53ca\u6ce8\u610f\u4e8b\u9805\uff0c\u63d0\u5347\u6578\u64da\u7d71\u8a08\u6548\u7387\uff0c\u907f\u514d\u5e38\u898b\u932f\u8aa4\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-186266","post","type-post","status-publish","format-standard","hentry","category-database"],"_links":{"self":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/186266","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=186266"}],"version-history":[{"count":1,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/186266\/revisions"}],"predecessor-version":[{"id":186267,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/186266\/revisions\/186267"}],"wp:attachment":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/media?parent=186266"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/categories?post=186266"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/tags?post=186266"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}