{"id":74538,"date":"2024-10-15T22:34:11","date_gmt":"2024-10-15T14:34:11","guid":{"rendered":"https:\/\/server.hk\/cnblog\/74538\/"},"modified":"2024-10-15T22:34:11","modified_gmt":"2024-10-15T14:34:11","slug":"mysql-left-join-%e8%81%af%e5%90%88%e6%9f%a5%e8%a9%a2%e7%9a%84%e6%95%88%e7%8e%87%e5%88%86%e6%9e%90","status":"publish","type":"post","link":"https:\/\/server.hk\/cnblog\/74538\/","title":{"rendered":"MySQL LEFT JOIN \u806f\u5408\u67e5\u8a62\u7684\u6548\u7387\u5206\u6790"},"content":{"rendered":"<h1 id=\"mysql-left-join-%e8%81%af%e5%90%88%e6%9f%a5%e8%a9%a2%e7%9a%84%e6%95%88%e7%8e%87%e5%88%86%e6%9e%90-PmAIxevlGN\">MySQL LEFT JOIN \u806f\u5408\u67e5\u8a62\u7684\u6548\u7387\u5206\u6790<\/h1>\n<p>\u5728\u6578\u64da\u5eab\u7ba1\u7406\u7cfb\u7d71\u4e2d\uff0cMySQL \u662f\u4e00\u500b\u5ee3\u6cdb\u4f7f\u7528\u7684\u958b\u6e90\u6578\u64da\u5eab\uff0c\u7279\u5225\u662f\u5728\u7db2\u9801\u61c9\u7528\u7a0b\u5f0f\u4e2d\u3002\u7576\u6d89\u53ca\u5230\u591a\u500b\u8868\u7684\u6578\u64da\u67e5\u8a62\u6642\uff0cJOIN \u64cd\u4f5c\u662f\u4e0d\u53ef\u6216\u7f3a\u7684\u3002\u672c\u6587\u5c07\u91cd\u9ede\u5206\u6790 MySQL \u4e2d\u7684 LEFT JOIN \u806f\u5408\u67e5\u8a62\u7684\u6548\u7387\uff0c\u4e26\u63d0\u4f9b\u4e00\u4e9b\u6700\u4f73\u5be6\u8e10\u548c\u512a\u5316\u5efa\u8b70\u3002<\/p>\n<h2 id=\"%e4%bb%80%e9%ba%bc%e6%98%af-left-join%ef%bc%9f-PmAIxevlGN\">\u4ec0\u9ebc\u662f LEFT JOIN\uff1f<\/h2>\n<p>LEFT JOIN \u662f\u4e00\u7a2e\u806f\u5408\u67e5\u8a62\uff0c\u4e3b\u8981\u7528\u65bc\u5f9e\u5de6\u5074\u8868\u4e2d\u9078\u53d6\u6240\u6709\u8a18\u9304\uff0c\u5373\u4f7f\u5728\u53f3\u5074\u8868\u4e2d\u6c92\u6709\u5339\u914d\u7684\u8a18\u9304\u3002\u9019\u610f\u5473\u8457\uff0c\u5982\u679c\u5de6\u5074\u8868\u4e2d\u7684\u67d0\u4e00\u884c\u5728\u53f3\u5074\u8868\u4e2d\u627e\u4e0d\u5230\u5c0d\u61c9\u7684\u884c\uff0c\u5247\u7d50\u679c\u4e2d\u4ecd\u6703\u986f\u793a\u8a72\u884c\uff0c\u4f46\u53f3\u5074\u8868\u7684\u6b04\u4f4d\u5c07\u986f\u793a\u70ba NULL\u3002<\/p>\n<pre><code>SELECT a.*, b.*\nFROM table_a AS a\nLEFT JOIN table_b AS b ON a.id = b.a_id;\n<\/code><\/pre>\n<p>\u5728\u9019\u500b\u4f8b\u5b50\u4e2d\uff0c\u67e5\u8a62\u5c07\u8fd4\u56de table_a \u4e2d\u7684\u6240\u6709\u884c\uff0c\u4e26\u4e14\u5982\u679c table_b \u4e2d\u6709\u5339\u914d\u7684\u884c\uff0c\u5247\u6703\u986f\u793a\u76f8\u61c9\u7684\u6578\u64da\uff1b\u5982\u679c\u6c92\u6709\u5339\u914d\uff0c\u5247 table_b \u7684\u6b04\u4f4d\u5c07\u986f\u793a\u70ba NULL\u3002<\/p>\n<h2 id=\"left-join-%e7%9a%84%e6%95%88%e7%8e%87%e8%80%83%e9%87%8f-PmAIxevlGN\">LEFT JOIN \u7684\u6548\u7387\u8003\u91cf<\/h2>\n<p>\u5728\u9032\u884c LEFT JOIN \u67e5\u8a62\u6642\uff0c\u6548\u7387\u662f\u958b\u767c\u8005\u9700\u8981\u8003\u616e\u7684\u91cd\u8981\u56e0\u7d20\u3002\u4ee5\u4e0b\u662f\u5f71\u97ff LEFT JOIN \u6548\u7387\u7684\u5e7e\u500b\u95dc\u9375\u56e0\u7d20\uff1a<\/p>\n<h3 id=\"1-%e8%a1%a8%e7%9a%84%e5%a4%a7%e5%b0%8f-PmAIxevlGN\">1. \u8868\u7684\u5927\u5c0f<\/h3>\n<p>\u8868\u7684\u5927\u5c0f\u76f4\u63a5\u5f71\u97ff\u67e5\u8a62\u7684\u6027\u80fd\u3002\u7576\u5de6\u5074\u8868\u6216\u53f3\u5074\u8868\u7684\u8a18\u9304\u6578\u91cf\u975e\u5e38\u9f90\u5927\u6642\uff0c\u67e5\u8a62\u7684\u57f7\u884c\u6642\u9593\u6703\u986f\u8457\u589e\u52a0\u3002\u70ba\u4e86\u63d0\u9ad8\u6548\u7387\uff0c\u5efa\u8b70\u5c0d\u8868\u9032\u884c\u9069\u7576\u7684\u7d22\u5f15\u3002<\/p>\n<h3 id=\"2-%e7%b4%a2%e5%bc%95%e7%9a%84%e4%bd%bf%e7%94%a8-PmAIxevlGN\">2. \u7d22\u5f15\u7684\u4f7f\u7528<\/h3>\n<p>\u5728\u9032\u884c JOIN \u64cd\u4f5c\u6642\uff0c\u4f7f\u7528\u7d22\u5f15\u53ef\u4ee5\u986f\u8457\u63d0\u9ad8\u67e5\u8a62\u6027\u80fd\u3002\u78ba\u4fdd\u5728 JOIN \u689d\u4ef6\u4e2d\u4f7f\u7528\u7684\u6b04\u4f4d\u4e0a\u5efa\u7acb\u7d22\u5f15\uff0c\u9019\u6a23\u53ef\u4ee5\u52a0\u5feb\u67e5\u8a62\u901f\u5ea6\u3002\u4f8b\u5982\uff1a<\/p>\n<pre><code>CREATE INDEX idx_a_id ON table_b(a_id);\n<\/code><\/pre>\n<p>\u9019\u6a23\u53ef\u4ee5\u52a0\u901f table_b \u4e2d a_id \u6b04\u4f4d\u7684\u67e5\u8a62\uff0c\u5f9e\u800c\u63d0\u9ad8 LEFT JOIN \u7684\u6548\u7387\u3002<\/p>\n<h3 id=\"3-%e6%9f%a5%e8%a9%a2%e7%9a%84%e8%a4%87%e9%9b%9c%e6%80%a7-PmAIxevlGN\">3. \u67e5\u8a62\u7684\u8907\u96dc\u6027<\/h3>\n<p>\u67e5\u8a62\u7684\u8907\u96dc\u6027\u4e5f\u6703\u5f71\u97ff\u6027\u80fd\u3002\u904e\u591a\u7684 JOIN \u64cd\u4f5c\u6703\u5c0e\u81f4\u67e5\u8a62\u8b8a\u5f97\u7de9\u6162\u3002\u5efa\u8b70\u5728\u8a2d\u8a08\u6578\u64da\u5eab\u6642\uff0c\u76e1\u91cf\u6e1b\u5c11\u4e0d\u5fc5\u8981\u7684 JOIN\uff0c\u4e26\u8003\u616e\u4f7f\u7528\u5b50\u67e5\u8a62\u6216\u5176\u4ed6\u65b9\u6cd5\u4f86\u7c21\u5316\u67e5\u8a62\u3002<\/p>\n<h2 id=\"%e6%9c%80%e4%bd%b3%e5%af%a6%e8%b8%90-PmAIxevlGN\">\u6700\u4f73\u5be6\u8e10<\/h2>\n<ul>\n<li><strong>\u4f7f\u7528\u9069\u7576\u7684\u7d22\u5f15\uff1a<\/strong>\u78ba\u4fdd\u5728 JOIN \u7684\u6b04\u4f4d\u4e0a\u5efa\u7acb\u7d22\u5f15\uff0c\u4ee5\u63d0\u9ad8\u67e5\u8a62\u6027\u80fd\u3002<\/li>\n<li><strong>\u9650\u5236\u8fd4\u56de\u7684\u6b04\u4f4d\uff1a<\/strong>\u53ea\u9078\u64c7\u9700\u8981\u7684\u6b04\u4f4d\uff0c\u800c\u4e0d\u662f\u4f7f\u7528 SELECT *\uff0c\u9019\u6a23\u53ef\u4ee5\u6e1b\u5c11\u6578\u64da\u50b3\u8f38\u91cf\u3002<\/li>\n<li><strong>\u8003\u616e\u6578\u64da\u5eab\u8a2d\u8a08\uff1a<\/strong>\u5728\u8a2d\u8a08\u6578\u64da\u5eab\u6642\uff0c\u8003\u616e\u5230\u672a\u4f86\u7684\u67e5\u8a62\u9700\u6c42\uff0c\u5408\u7406\u898f\u5283\u8868\u7684\u7d50\u69cb\u3002<\/li>\n<li><strong>\u5b9a\u671f\u512a\u5316\u6578\u64da\u5eab\uff1a<\/strong>\u5b9a\u671f\u9032\u884c\u6578\u64da\u5eab\u7684\u512a\u5316\u548c\u7dad\u8b77\uff0c\u4ee5\u78ba\u4fdd\u67e5\u8a62\u6027\u80fd\u3002<\/li>\n<\/ul>\n<h2 id=\"%e7%b5%90%e8%ab%96-PmAIxevlGN\">\u7d50\u8ad6<\/h2>\n<p>MySQL \u7684 LEFT JOIN \u662f\u4e00\u500b\u5f37\u5927\u7684\u5de5\u5177\uff0c\u53ef\u4ee5\u6709\u6548\u5730\u5f9e\u591a\u500b\u8868\u4e2d\u63d0\u53d6\u6578\u64da\u3002\u7136\u800c\uff0c\u70ba\u4e86\u78ba\u4fdd\u67e5\u8a62\u7684\u6548\u7387\uff0c\u958b\u767c\u8005\u9700\u8981\u8003\u616e\u8868\u7684\u5927\u5c0f\u3001\u7d22\u5f15\u7684\u4f7f\u7528\u4ee5\u53ca\u67e5\u8a62\u7684\u8907\u96dc\u6027\u7b49\u56e0\u7d20\u3002\u901a\u904e\u9075\u5faa\u6700\u4f73\u5be6\u8e10\uff0c\u53ef\u4ee5\u986f\u8457\u63d0\u9ad8 LEFT JOIN \u67e5\u8a62\u7684\u6027\u80fd\u3002<\/p>\n<p>\u5982\u679c\u60a8\u6b63\u5728\u5c0b\u627e\u9ad8\u6548\u7684 <a href=\"https:\/\/server.hk\">VPS<\/a> \u89e3\u6c7a\u65b9\u6848\u4f86\u652f\u6301\u60a8\u7684\u6578\u64da\u5eab\u61c9\u7528\uff0cServer.HK \u63d0\u4f9b\u591a\u7a2e\u9078\u64c7\uff0c\u6eff\u8db3\u4e0d\u540c\u9700\u6c42\u7684\u5ba2\u6236\u3002\u7121\u8ad6\u662f <a href=\"https:\/\/server.hk\">\u9999\u6e2f\u4f3a\u670d\u5668<\/a> \u9084\u662f <a href=\"https:\/\/server.hk\">\u4e91\u4f3a\u670d\u5668<\/a>\uff0c\u6211\u5011\u90fd\u80fd\u70ba\u60a8\u63d0\u4f9b\u7a69\u5b9a\u7684\u6027\u80fd\u548c\u53ef\u9760\u7684\u652f\u6301\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u5206\u6790MySQL LEFT JOIN\u806f\u5408\u67e5\u8a62\u7684\u6548\u7387\uff0c\u63a2\u8a0e\u6027\u80fd\u5f71\u97ff\u56e0\u7d20\u53ca\u6700\u4f73\u5be6\u8e10\uff0c\u63d0\u5347\u6578\u64da\u67e5\u8a62\u6548\u80fd\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-74538","post","type-post","status-publish","format-standard","hentry","category-database"],"_links":{"self":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/74538","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=74538"}],"version-history":[{"count":1,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/74538\/revisions"}],"predecessor-version":[{"id":74544,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/posts\/74538\/revisions\/74544"}],"wp:attachment":[{"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/media?parent=74538"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/categories?post=74538"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/server.hk\/cnblog\/wp-json\/wp\/v2\/tags?post=74538"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}