{"id":187714,"date":"2024-11-11T16:34:17","date_gmt":"2024-11-11T08:34:17","guid":{"rendered":"https:\/\/server.hk\/cnblog\/187714\/"},"modified":"2024-11-11T16:34:18","modified_gmt":"2024-11-11T08:34:18","slug":"%e6%95%b8%e6%93%9a%e5%ba%ab%e4%b8%ad%e7%9a%84count%e5%87%bd%e6%95%b8%e5%a6%82%e4%bd%95%e4%bd%bf%e7%94%a8%ef%bc%9f-%e6%95%b8%e6%93%9a%e5%ba%abcount%e5%87%bd%e6%95%b8","status":"publish","type":"post","link":"https:\/\/server.hk\/cnblog\/187714\/","title":{"rendered":"\u6578\u64da\u5eab\u4e2d\u7684Count\u51fd\u6578\u5982\u4f55\u4f7f\u7528\uff1f (\u6578\u64da\u5eabCount\u51fd\u6578)"},"content":{"rendered":"<h1 id=\"%e6%95%b8%e6%93%9a%e5%ba%ab%e4%b8%ad%e7%9a%84count%e5%87%bd%e6%95%b8%e5%a6%82%e4%bd%95%e4%bd%bf%e7%94%a8%ef%bc%9f-%e6%95%b8%e6%93%9a%e5%ba%abcount%e5%87%bd%e6%95%b8-fqLWCyBgEU\">\u6578\u64da\u5eab\u4e2d\u7684Count\u51fd\u6578\u5982\u4f55\u4f7f\u7528\uff1f (\u6578\u64da\u5eabCount\u51fd\u6578)<\/h1>\n<p>\u5728\u6578\u64da\u5eab\u7ba1\u7406\u7cfb\u7d71\u4e2d\uff0c<code>COUNT<\/code> \u51fd\u6578\u662f\u4e00\u500b\u975e\u5e38\u91cd\u8981\u7684\u805a\u5408\u51fd\u6578\uff0c\u7528\u65bc\u8a08\u7b97\u67e5\u8a62\u7d50\u679c\u4e2d\u884c\u7684\u6578\u91cf\u3002\u7121\u8ad6\u662f\u5728\u6578\u64da\u5206\u6790\u3001\u5831\u544a\u751f\u6210\u9084\u662f\u6578\u64da\u6316\u6398\u4e2d\uff0c<code>COUNT<\/code> \u51fd\u6578\u90fd\u626e\u6f14\u8457\u4e0d\u53ef\u6216\u7f3a\u7684\u89d2\u8272\u3002\u672c\u6587\u5c07\u6df1\u5165\u63a2\u8a0e <code>COUNT<\/code> \u51fd\u6578\u7684\u4f7f\u7528\u65b9\u6cd5\u3001\u8a9e\u6cd5\u53ca\u5176\u5728\u5be6\u969b\u61c9\u7528\u4e2d\u7684\u793a\u4f8b\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-fqLWCyBgEU\">COUNT\u51fd\u6578\u7684\u57fa\u672c\u8a9e\u6cd5<\/h2>\n<p><code>COUNT<\/code> \u51fd\u6578\u7684\u57fa\u672c\u8a9e\u6cd5\u5982\u4e0b\uff1a<\/p>\n<pre><code>COUNT(expression)<\/code><\/pre>\n<p>\u5176\u4e2d\uff0c<code>expression<\/code> \u53ef\u4ee5\u662f\u4efb\u4f55\u5217\u540d\u6216\u8868\u9054\u5f0f\u3002<code>COUNT<\/code> \u51fd\u6578\u6703\u8fd4\u56de\u7b26\u5408\u689d\u4ef6\u7684\u884c\u6578\u3002\u9700\u8981\u6ce8\u610f\u7684\u662f\uff0c<code>COUNT(*)<\/code> \u548c <code>COUNT(column_name)<\/code> \u4e4b\u9593\u5b58\u5728\u4e00\u4e9b\u5dee\u7570\u3002<\/p>\n<h2 id=\"count-%e8%88%87-countcolumn_name-%e7%9a%84%e5%8d%80%e5%88%a5-fqLWCyBgEU\">COUNT(*) \u8207 COUNT(column_name) \u7684\u5340\u5225<\/h2>\n<ul>\n<li><strong>COUNT(*)\uff1a<\/strong>\u8a08\u7b97\u8868\u4e2d\u6240\u6709\u884c\u7684\u6578\u91cf\uff0c\u5305\u62ec NULL \u503c\u3002<\/li>\n<li><strong>COUNT(column_name)\uff1a<\/strong>\u50c5\u8a08\u7b97\u6307\u5b9a\u5217\u4e2d\u975e NULL \u503c\u7684\u6578\u91cf\u3002<\/li>\n<\/ul>\n<p>\u4f8b\u5982\uff0c\u5047\u8a2d\u6709\u4e00\u500b\u540d\u70ba <code>employees<\/code> \u7684\u8868\uff0c\u5305\u542b\u4ee5\u4e0b\u6578\u64da\uff1a<\/p>\n<pre><code>\n| id | name   | department |\n|----|--------|------------|\n| 1  | Alice  | HR         |\n| 2  | Bob    | IT         |\n| 3  | Charlie| NULL       |\n| 4  | David  | IT         |\n<\/code><\/pre>\n<p>\u4f7f\u7528 <code>COUNT(*)<\/code> \u548c <code>COUNT(department)<\/code> \u7684\u67e5\u8a62\u7d50\u679c\u5982\u4e0b\uff1a<\/p>\n<pre><code>\nSELECT COUNT(*) FROM employees; -- \u8fd4\u56de 4\nSELECT COUNT(department) FROM employees; -- \u8fd4\u56de 3\n<\/code><\/pre>\n<h2 id=\"%e4%bd%bf%e7%94%a8count%e5%87%bd%e6%95%b8%e7%9a%84%e5%af%a6%e9%9a%9b%e7%a4%ba%e4%be%8b-fqLWCyBgEU\">\u4f7f\u7528COUNT\u51fd\u6578\u7684\u5be6\u969b\u793a\u4f8b<\/h2>\n<p>\u4ee5\u4e0b\u662f\u4e00\u4e9b\u4f7f\u7528 <code>COUNT<\/code> \u51fd\u6578\u7684\u5be6\u969b\u793a\u4f8b\uff1a<\/p>\n<h3 id=\"%e7%a4%ba%e4%be%8b1%ef%bc%9a%e8%a8%88%e7%ae%97%e7%89%b9%e5%ae%9a%e9%83%a8%e9%96%80%e7%9a%84%e5%93%a1%e5%b7%a5%e6%95%b8%e9%87%8f-fqLWCyBgEU\">\u793a\u4f8b1\uff1a\u8a08\u7b97\u7279\u5b9a\u90e8\u9580\u7684\u54e1\u5de5\u6578\u91cf<\/h3>\n<p>\u5982\u679c\u6211\u5011\u60f3\u8981\u8a08\u7b97 IT \u90e8\u9580\u7684\u54e1\u5de5\u6578\u91cf\uff0c\u53ef\u4ee5\u4f7f\u7528\u4ee5\u4e0b\u67e5\u8a62\uff1a<\/p>\n<pre><code>\nSELECT COUNT(*) FROM employees WHERE department = 'IT';\n<\/code><\/pre>\n<p>\u9019\u5c07\u8fd4\u56de IT \u90e8\u9580\u7684\u54e1\u5de5\u6578\u91cf\u3002<\/p>\n<h3 id=\"%e7%a4%ba%e4%be%8b2%ef%bc%9a%e5%88%86%e7%b5%84%e8%a8%88%e7%ae%97%e6%af%8f%e5%80%8b%e9%83%a8%e9%96%80%e7%9a%84%e5%93%a1%e5%b7%a5%e6%95%b8%e9%87%8f-fqLWCyBgEU\">\u793a\u4f8b2\uff1a\u5206\u7d44\u8a08\u7b97\u6bcf\u500b\u90e8\u9580\u7684\u54e1\u5de5\u6578\u91cf<\/h3>\n<p>\u5982\u679c\u6211\u5011\u60f3\u8981\u6309\u90e8\u9580\u5206\u7d44\u8a08\u7b97\u6bcf\u500b\u90e8\u9580\u7684\u54e1\u5de5\u6578\u91cf\uff0c\u53ef\u4ee5\u4f7f\u7528 <code>GROUP BY<\/code> \u5b50\u53e5\uff1a<\/p>\n<pre><code>\nSELECT department, COUNT(*) as employee_count \nFROM employees \nGROUP BY department;\n<\/code><\/pre>\n<p>\u9019\u5c07\u8fd4\u56de\u6bcf\u500b\u90e8\u9580\u53ca\u5176\u5c0d\u61c9\u7684\u54e1\u5de5\u6578\u91cf\u3002<\/p>\n<h3 id=\"%e7%a4%ba%e4%be%8b3%ef%bc%9a%e8%a8%88%e7%ae%97%e4%b8%8d%e5%90%8c%e6%a2%9d%e4%bb%b6%e4%b8%8b%e7%9a%84%e8%a1%8c%e6%95%b8-fqLWCyBgEU\">\u793a\u4f8b3\uff1a\u8a08\u7b97\u4e0d\u540c\u689d\u4ef6\u4e0b\u7684\u884c\u6578<\/h3>\n<p>\u6211\u5011\u9084\u53ef\u4ee5\u4f7f\u7528 <code>COUNT<\/code> \u51fd\u6578\u4f86\u8a08\u7b97\u7b26\u5408\u7279\u5b9a\u689d\u4ef6\u7684\u884c\u6578\u3002\u4f8b\u5982\uff0c\u8a08\u7b97\u6240\u6709\u975e NULL \u54e1\u5de5\u59d3\u540d\u7684\u6578\u91cf\uff1a<\/p>\n<pre><code>\nSELECT COUNT(name) FROM employees WHERE name IS NOT NULL;\n<\/code><\/pre>\n<h2 id=\"%e7%b8%bd%e7%b5%90-fqLWCyBgEU\">\u7e3d\u7d50<\/h2>\n<p>\u7e3d\u7684\u4f86\u8aaa\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\u5eab\u4e2d\u884c\u7684\u6578\u91cf\u3002\u7121\u8ad6\u662f\u8a08\u7b97\u6574\u500b\u8868\u7684\u884c\u6578\uff0c\u9084\u662f\u6839\u64da\u7279\u5b9a\u689d\u4ef6\u9032\u884c\u8a08\u7b97\uff0c<code>COUNT<\/code> \u51fd\u6578\u90fd\u80fd\u63d0\u4f9b\u6e96\u78ba\u7684\u7d50\u679c\u3002\u5c0d\u65bc\u9700\u8981\u9032\u884c\u6578\u64da\u5206\u6790\u548c\u5831\u544a\u7684\u7528\u6236\u4f86\u8aaa\uff0c\u638c\u63e1 <code>COUNT<\/code> \u51fd\u6578\u7684\u4f7f\u7528\u662f\u975e\u5e38\u91cd\u8981\u7684\u3002<\/p>\n<p>\u5982\u679c\u60a8\u5c0d\u65bc\u6578\u64da\u5eab\u7ba1\u7406\u6216\u5176\u4ed6\u76f8\u95dc\u6280\u8853\u6709\u8208\u8da3\uff0c\u6b61\u8fce\u8a2a\u554f\u6211\u5011\u7684\u7db2\u7ad9\u4e86\u89e3\u66f4\u591a\u8cc7\u8a0a\uff0c\u7279\u5225\u662f\u6211\u5011\u63d0\u4f9b\u7684 <a href=\"https:\/\/server.hk\">\u9999\u6e2fVPS<\/a> \u670d\u52d9\uff0c\u80fd\u5920\u70ba\u60a8\u7684\u6578\u64da\u5eab\u61c9\u7528\u63d0\u4f9b\u7a69\u5b9a\u7684\u652f\u6301\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u4e86\u89e3\u6578\u64da\u5eab\u4e2d\u7684Count\u51fd\u6578\u5982\u4f55\u4f7f\u7528\uff0c\u8a08\u7b97\u8a18\u9304\u6578\u91cf\uff0c\u63d0\u5347\u6578\u64da\u67e5\u8a62\u6548\u7387\uff0c\u7c21\u5316\u6578\u64da\u5206\u6790\u904e\u7a0b\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-187714","post","type-post","status-publish","format-standard","hentry","category-database"],"_links":{"self":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/187714","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=187714"}],"version-history":[{"count":1,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/187714\/revisions"}],"predecessor-version":[{"id":187715,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/187714\/revisions\/187715"}],"wp:attachment":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/media?parent=187714"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/categories?post=187714"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/tags?post=187714"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}