MySQL去重保留最大的那条记录（取最新的记录） - 后端笔记

数据表：

SQL Code复制内容到剪贴板

-- ----------------------------
-- Table structure for t_login_log
-- ----------------------------
CREATE TABLE `t_login_log` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user_id` int(11) NOT NULL COMMENT '用户ID',
`device_name` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL COMMENT '登录设备',
`login_time` datetime DEFAULT NULL COMMENT '登录时间',
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
-- ----------------------------
-- Records of t_login_log
-- ----------------------------
INSERT INTO `t_login_log` VALUES (1, 1121, 'iPhone 6s', '2019-07-01 19:20:25');
INSERT INTO `t_login_log` VALUES (2, 2120, 'vivo x20', '2019-06-28 16:21:11');
INSERT INTO `t_login_log` VALUES (3, 1607, 'huawei P30', '2019-07-04 19:21:59');
INSERT INTO `t_login_log` VALUES (4, 2120, 'vivo x20', '2019-06-30 19:22:34');
INSERT INTO `t_login_log` VALUES (5, 2120, 'vivo x20', '2019-07-04 19:23:07');
INSERT INTO `t_login_log` VALUES (6, 1121, 'iPad mini', '2019-07-03 19:23:25');
INSERT INTO `t_login_log` VALUES (7, 1607, 'iPhone 8 Plus', '2019-06-30 19:24:06');
INSERT INTO `t_login_log` VALUES (8, 1970, 'MI8', '2019-07-03 19:25:00');

以用户登录日志表为例，取用户最近登录的设备，自连接，取最新的记录

SQL Code复制内容到剪贴板

SELECT * FROM t_login_log ORDER BY user_id;
SELECT
t1.*
FROM t_login_log t1
LEFT JOIN t_login_log t2 ON t1.user_id = t2.user_id AND t1.login_time < t2.login_time
WHERE t2.id IS NULL;

应该获取的记录是以下红框内的数据

第二种：

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SELECT t.* FROM (SELECT id,user_id,max(login_time) AS login_time FROM t_login_log GROUP BY user_id) a
LEFT JOIN t_login_log t
ON t.user_id=a.user_id AND t.login_time=a.login_time

第三种，不需要关联到ID的情况下，只取最大login_time

SQL Code复制内容到剪贴板