SQL 实例

---

Date: 2020-08-24 16:07:35
*/

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------
-- Table structure for websites
-- ----------------------------
DROP TABLE IF EXISTS `websites`;
CREATE TABLE `websites` (
  `id` int(255) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL COMMENT '网名',
  `url` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL COMMENT '网址',
  `alexa` int(255) DEFAULT NULL COMMENT '排名',
  `country` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL COMMENT '城市',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=18 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of websites
-- ----------------------------
INSERT INTO `websites` VALUES ('1', 'Google', 'https://www.google.cm/', '1', 'USA');
INSERT INTO `websites` VALUES ('2', '淘宝', 'https://www.taobao.com/', '13', 'CN');
INSERT INTO `websites` VALUES ('3', '菜鸟教程', 'http://www.runoob.com/', '4689', 'CN');
INSERT INTO `websites` VALUES ('4', '微博 ', 'http://weibo.com/', '20', 'CN');
INSERT INTO `websites` VALUES ('5', 'Facebook', 'https://www.facebook.com/', '3', 'USA');
INSERT INTO `websites` VALUES ('7', 'stackoverflow', 'http://stackoverflow.com/', '0', 'IND');

Date: 2020-08-24 15:58:02
*/

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------
-- Table structure for access_log
-- ----------------------------
DROP TABLE IF EXISTS `access_log`;
CREATE TABLE `access_log` (
  `aid` varchar(255) NOT NULL,
  `site_id` int(255) DEFAULT NULL,
  `count` int(255) DEFAULT NULL,
  `date` date DEFAULT NULL,
  PRIMARY KEY (`aid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of access_log
-- ----------------------------
INSERT INTO `access_log` VALUES ('1', '1', '45', '2016-05-10');
INSERT INTO `access_log` VALUES ('2', '3', '100', '2016-05-13');
INSERT INTO `access_log` VALUES ('3', '1', '230', '2016-05-14');
INSERT INTO `access_log` VALUES ('4', '2', '10', '2016-05-14');
INSERT INTO `access_log` VALUES ('5', '5', '205', '2016-05-14');
INSERT INTO `access_log` VALUES ('6', '4', '13', '2016-05-15');
INSERT INTO `access_log` VALUES ('7', '3', '220', '2016-05-15');
INSERT INTO `access_log` VALUES ('8', '5', '545', '2016-05-16');
INSERT INTO `access_log` VALUES ('9', '3', '201', '2016-05-17');

SELECT
    w.`name`,
    al.count,
    al.date
FROM
    websites AS w
LEFT JOIN access_log AS al ON w.id = al.site_id
ORDER BY
    al.count DESC;

By:LEFT JOIN 关键字从左表（table1）返回所有的行，即使右表（table2）中没有匹配。如果右表中没有匹配，则结果为 NULL。

 原文链接转自：https://www.runoob.com/sql/sql-join-left.html

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