{"id":973,"date":"2025-01-15T00:43:50","date_gmt":"2025-01-14T16:43:50","guid":{"rendered":"https:\/\/www.laixuexila.com\/?p=973"},"modified":"2025-01-15T00:43:50","modified_gmt":"2025-01-14T16:43:50","slug":"ql-%e4%b8%ad-left-join-%e5%92%8c-right-join-%e6%9f%a5%e8%af%a2%e4%bd%95%e6%97%b6%e5%8f%af%e4%bb%a5%e4%ba%92%e6%8d%a2%ef%bc%9f","status":"publish","type":"post","link":"https:\/\/www.laixuexila.com\/index.php\/2025\/01\/15\/ql-%e4%b8%ad-left-join-%e5%92%8c-right-join-%e6%9f%a5%e8%af%a2%e4%bd%95%e6%97%b6%e5%8f%af%e4%bb%a5%e4%ba%92%e6%8d%a2%ef%bc%9f\/","title":{"rendered":"QL \u4e2d LEFT JOIN \u548c RIGHT JOIN \u67e5\u8be2\u4f55\u65f6\u53ef\u4ee5\u4e92\u6362\uff1f"},"content":{"rendered":"\n
\n

\u5728 SQL \u4e2d\uff0cLEFT JOIN<\/code> \u548c RIGHT JOIN<\/code> \u90fd\u662f\u7528\u4e8e\u8054\u63a5\u4e24\u4e2a\u8868\u7684\u7c7b\u578b\u3002\u5b83\u4eec\u7684\u529f\u80fd\u76f8\u4f3c\uff0c\u4f46\u5728\u9ed8\u8ba4\u884c\u4e3a\u4e0a\u6709\u6240\u4e0d\u540c\u3002\u7406\u89e3\u8fd9\u4e24\u8005\u7684\u533a\u522b\uff0c\u5e76\u4e86\u89e3\u4f55\u65f6\u53ef\u4ee5\u4e92\u6362\u5b83\u4eec\uff0c\u6709\u52a9\u4e8e\u4f18\u5316\u67e5\u8be2\u548c\u7406\u89e3\u4e0d\u540c\u7684\u8054\u63a5\u65b9\u5f0f\u3002<\/p>\n<\/blockquote>\n\n\n\n

1. LEFT JOIN \u548c RIGHT JOIN \u57fa\u672c\u6982\u5ff5<\/strong><\/h3>\n\n\n\n
    \n
  • LEFT JOIN<\/strong>\uff08\u6216\u79f0\u4e3a LEFT OUTER JOIN<\/strong>\uff09\uff1a\u8fd4\u56de\u5de6\u8868\uff08LEFT JOIN<\/code> \u4e2d\u7684\u7b2c\u4e00\u4e2a\u8868\uff09\u7684\u6240\u6709\u8bb0\u5f55\uff0c\u5373\u4f7f\u5728\u53f3\u8868\u4e2d\u6ca1\u6709\u5339\u914d\u7684\u8bb0\u5f55\u3002\u5bf9\u4e8e\u53f3\u8868\u4e2d\u6ca1\u6709\u5339\u914d\u7684\u8bb0\u5f55\uff0c\u67e5\u8be2\u7ed3\u679c\u5c06\u8fd4\u56de NULL<\/code>\u3002<\/li>\n\n\n\n
  • RIGHT JOIN<\/strong>\uff08\u6216\u79f0\u4e3a RIGHT OUTER JOIN<\/strong>\uff09\uff1a\u8fd4\u56de\u53f3\u8868\uff08RIGHT JOIN<\/code> \u4e2d\u7684\u7b2c\u4e8c\u4e2a\u8868\uff09\u7684\u6240\u6709\u8bb0\u5f55\uff0c\u5373\u4f7f\u5728\u5de6\u8868\u4e2d\u6ca1\u6709\u5339\u914d\u7684\u8bb0\u5f55\u3002\u5bf9\u4e8e\u5de6\u8868\u4e2d\u6ca1\u6709\u5339\u914d\u7684\u8bb0\u5f55\uff0c\u67e5\u8be2\u7ed3\u679c\u5c06\u8fd4\u56de NULL<\/code>\u3002<\/li>\n<\/ul>\n\n\n\n

    2. LEFT JOIN<\/code> \u548c RIGHT JOIN<\/code> \u4e4b\u95f4\u7684\u5173\u7cfb<\/strong><\/h3>\n\n\n\n

    LEFT JOIN<\/code> \u548c RIGHT JOIN<\/code> \u4e4b\u95f4\u7684\u5173\u7cfb\u662f\u5bf9\u79f0\u7684\uff0c\u610f\u601d\u662f\u5b83\u4eec\u80fd\u901a\u8fc7\u4ea4\u6362\u5de6\u53f3\u8868\u7684\u4f4d\u7f6e\u6765\u5b9e\u73b0\u76f8\u540c\u7684\u7ed3\u679c\u3002\u8fd9\u662f\u56e0\u4e3a\uff1a<\/p>\n\n\n\n

      \n
    • LEFT JOIN<\/code> \u4f1a\u4fdd\u7559\u5de6\u8868\u7684\u6240\u6709\u8bb0\u5f55\uff0c\u5373\u4f7f\u53f3\u8868\u6ca1\u6709\u5339\u914d\u9879\u3002<\/li>\n\n\n\n
    • RIGHT JOIN<\/code> \u4f1a\u4fdd\u7559\u53f3\u8868\u7684\u6240\u6709\u8bb0\u5f55\uff0c\u5373\u4f7f\u5de6\u8868\u6ca1\u6709\u5339\u914d\u9879\u3002<\/li>\n<\/ul>\n\n\n\n

      \u7531\u4e8e SQL \u4e2d\u7684\u8054\u63a5\u662f\u57fa\u4e8e\u4e24\u4e2a\u8868\u4e4b\u95f4\u7684\u5173\u7cfb\u8fdb\u884c\u7684\uff0c\u4f60\u53ef\u4ee5\u901a\u8fc7\u4ea4\u6362\u67e5\u8be2\u4e2d\u7684\u5de6\u53f3\u8868\u6765\u8f6c\u6362 LEFT JOIN<\/code> \u548c RIGHT JOIN<\/code>\u3002<\/p>\n\n\n\n

      3. \u4f55\u65f6\u53ef\u4ee5\u4e92\u6362 LEFT JOIN<\/code> \u548c RIGHT JOIN<\/code>\uff1f<\/strong><\/h3>\n\n\n\n

      \u4f60\u53ef\u4ee5\u901a\u8fc7\u7b80\u5355\u7684\u4ea4\u6362 LEFT JOIN<\/code> \u548c RIGHT JOIN<\/code> \u4e2d\u8868\u7684\u4f4d\u7f6e\u6765\u4f7f\u4e24\u8005\u7684\u67e5\u8be2\u7ed3\u679c\u4fdd\u6301\u4e00\u81f4\u3002<\/p>\n\n\n\n

      \u5047\u8bbe\u6709\u4ee5\u4e0b\u67e5\u8be2\uff1a<\/strong><\/p>\n\n\n\n

        \n
      • \u4f7f\u7528 LEFT JOIN<\/code>\uff1a<\/li>\n<\/ul>\n\n\n\n
          SELECT a.*, b.*\n  FROM table1 a\n  LEFT JOIN table2 b ON a.id = b.id;<\/code><\/pre>\n\n\n\n
          \n
        • \u4f7f\u7528 RIGHT JOIN<\/code>\uff1a<\/li>\n<\/ul>\n\n\n\n
            SELECT a.*, b.*\n  FROM table1 a\n  RIGHT JOIN table2 b ON a.id = b.id;<\/code><\/pre>\n\n\n\n
          \u4f60\u53ef\u4ee5\u901a\u8fc7\u4ea4\u6362\u4e24\u4e2a\u8868\u7684\u4f4d\u7f6e\uff0c\u4f7f\u5f97 RIGHT JOIN<\/code> \u7684\u67e5\u8be2\u53d8\u4e3a LEFT JOIN<\/code>\uff0c\u53cd\u4e4b\u4ea6\u7136\u3002<\/p>\n\n\n\n
            \n
          • \u5c06\u53f3\u8868\uff08table2<\/code>\uff09\u6539\u4e3a\u5de6\u8868\uff0c\u5e76\u5c06\u5de6\u8868\uff08table1<\/code>\uff09\u6539\u4e3a\u53f3\u8868\uff0c\u67e5\u8be2\u7ed3\u679c\u4fdd\u6301\u4e00\u81f4\u3002<\/li>\n<\/ul>\n\n\n\n
            \u8f6c\u6362\u4e3a LEFT JOIN<\/code> \u540e\uff1a<\/strong><\/p>\n\n\n\n
            SELECT a.*, b.*\nFROM table2 b\nLEFT JOIN table1 a ON a.id = b.id;<\/code><\/pre>\n\n\n\n
            4. \u4e3e\u4f8b\u8bf4\u660e<\/strong><\/h3>\n\n\n\n
            \u5047\u8bbe\u6709\u4e24\u4e2a\u8868\uff1a<\/p>\n\n\n\n
            employees<\/code> \u8868<\/strong>\uff1a<\/p>\n\n\n\n
            employee_id<\/th>name<\/th><\/tr><\/thead>
            1<\/td>John<\/td><\/tr>
            2<\/td>Jane<\/td><\/tr>
            3<\/td>Jack<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
            departments<\/code> \u8868<\/strong>\uff1a<\/p>\n\n\n\n
            department_id<\/th>employee_id<\/th><\/tr><\/thead>
            101<\/td>1<\/td><\/tr>
            102<\/td>2<\/td><\/tr>
            103<\/td>4<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
            1. \u4f7f\u7528 LEFT JOIN<\/code> \u67e5\u8be2\uff1a<\/strong><\/p>\n\n\n\n
            SELECT e.name, d.department_id\nFROM employees e\nLEFT JOIN departments d ON e.employee_id = d.employee_id;<\/code><\/pre>\n\n\n\n
            \u7ed3\u679c\u5c06\u662f\uff1a<\/p>\n\n\n\n
            name<\/th>department_id<\/th><\/tr><\/thead>
            John<\/td>101<\/td><\/tr>
            Jane<\/td>102<\/td><\/tr>
            Jack<\/td>NULL<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
            2. \u4f7f\u7528 RIGHT JOIN<\/code> \u67e5\u8be2\uff1a<\/strong><\/p>\n\n\n\n
            SELECT e.name, d.department_id\nFROM employees e\nRIGHT JOIN departments d ON e.employee_id = d.employee_id;<\/code><\/pre>\n\n\n\n
            \u7ed3\u679c\u5c06\u662f\uff1a<\/p>\n\n\n\n
            name<\/th>department_id<\/th><\/tr><\/thead>
            John<\/td>101<\/td><\/tr>
            Jane<\/td>102<\/td><\/tr>
            NULL<\/td>103<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
            \u53ef\u4ee5\u4ea4\u6362\u8868\u7684\u987a\u5e8f\u5f97\u5230\u76f8\u540c\u7684\u7ed3\u679c<\/strong>\u3002\u5982\u679c\u4f60\u5c06 RIGHT JOIN<\/code> \u6539\u4e3a LEFT JOIN<\/code>\uff0c\u5e76\u4ea4\u6362\u8868\u7684\u987a\u5e8f\uff0c\u7ed3\u679c\u5c06\u4f1a\u4e0e\u4e0a\u8ff0\u67e5\u8be2\u76f8\u540c\u3002<\/p>\n\n\n\n
            SELECT e.name, d.department_id\nFROM departments d\nLEFT JOIN employees e ON e.employee_id = d.employee_id;<\/code><\/pre>\n\n\n\n
            5. \u603b\u7ed3<\/strong><\/h3>\n\n\n\n
              \n
            • LEFT JOIN<\/code> \u548c RIGHT JOIN<\/code> \u67e5\u8be2\u7ed3\u679c\u662f\u53ef\u4ee5\u4e92\u6362\u7684\uff0c\u53ea\u9700\u8981\u4ea4\u6362\u8868\u7684\u4f4d\u7f6e\u3002<\/li>\n\n\n\n
            • LEFT JOIN<\/code> \u4fdd\u7559\u5de6\u8868\u7684\u6240\u6709\u8bb0\u5f55\uff0cRIGHT JOIN<\/code> \u4fdd\u7559\u53f3\u8868\u7684\u6240\u6709\u8bb0\u5f55\u3002\u901a\u8fc7\u4ea4\u6362\u8868\u7684\u4f4d\u7f6e\uff0c\u53ef\u4ee5\u5728\u67e5\u8be2\u4e2d\u4f7f\u7528\u4e24\u8005\u3002<\/li>\n\n\n\n
            • \u5728\u67e5\u8be2\u4e2d\uff0c\u9009\u62e9 LEFT JOIN<\/code> \u8fd8\u662f RIGHT JOIN<\/code> \u53ef\u4ee5\u6839\u636e\u5b9e\u9645\u9700\u6c42\u548c\u8868\u7684\u4f4d\u7f6e\u6765\u51b3\u5b9a\uff0c\u901a\u5e38\u63a8\u8350\u4f7f\u7528 LEFT JOIN<\/code>\uff0c\u56e0\u4e3a\u5b83\u66f4\u7b26\u5408\u76f4\u89c2\u7406\u89e3\u3002<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"
              \u5728 SQL \u4e2d\uff0cLEFT JOIN \u548c RIGHT JOIN \u90fd\u662f\u7528\u4e8e\u8054\u63a5\u4e24\u4e2a\u8868\u7684\u7c7b\u578b\u3002\u5b83\u4eec\u7684\u529f\u80fd\u76f8\u4f3c\uff0c\u4f46\u5728\u9ed8 […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[35],"tags":[],"class_list":["post-973","post","type-post","status-publish","format-standard","hentry","category-mysql"],"_links":{"self":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/973","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/comments?post=973"}],"version-history":[{"count":1,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/973\/revisions"}],"predecessor-version":[{"id":974,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/973\/revisions\/974"}],"wp:attachment":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/media?parent=973"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/categories?post=973"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/tags?post=973"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}