mysql分组排序后，取前三

需求： 将数据按照用户进行分组，并根据城市和amount进行排序，取出每个城市排名前3的用户数据

测试数据文件：test.sql

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------
-- Table structure for test
-- ----------------------------
DROP TABLE IF EXISTS `test`;
CREATE TABLE `test` (
  `user_id` varchar(255) DEFAULT NULL,
  `city` varchar(255) DEFAULT NULL,
  `sex` varchar(255) DEFAULT NULL,
  `amount` float(255,0) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of test
-- ----------------------------
INSERT INTO `test` VALUES ('21e656ef-395d-4f9b-993a-dcc0e1a2db1a', '北京', '女', '4800');
INSERT INTO `test` VALUES ('2d165f95-43a4-402a-9c99-7817d583a199', '广州', '女', '23272');
INSERT INTO `test` VALUES ('0a274d08-453f-4cc3-9380-3fe74f6b7318', '北京', '女', '155480');
INSERT INTO `test` VALUES ('171f6549-9f88-4747-8a4a-33dcf1cb6527', '广州', '男', '80000');
INSERT INTO `test` VALUES ('127b017e-5af9-46dd-a15b-2974faa51d98', '上海', '女', '96000');
INSERT INTO `test` VALUES ('22be3b12-2e9d-46b8-93ab-ed3df13dcb74', '北京', '女', '36000');
INSERT INTO `test` VALUES ('266bfcd7-3ea9-4817-baf8-2ba96293b7d8', '广州', '男', '24000');
INSERT INTO `test` VALUES ('16c02230-7d5e-46f1-9ba5-cb0e34122395', '北京', '女', '320000');
INSERT INTO `test` VALUES ('18bb0ba2-966a-486e-81c5-f7821982a2ff', '上海', '男', '17600');
INSERT INTO `test` VALUES ('293424f8-a8cd-42eb-8b5a-7bc85fc25b9f', '北京', '女', '16838');
INSERT INTO `test` VALUES ('00b44351-ed41-4d5e-815f-a5319aa44575', '上海', '女', '160000');

实现结果：

SQL查询语句

SET @rownum := 0 ,@GROUP := ''; //初始化变量

SELECT
	user_id,
	sex,
	city,
	amount,
	r AS rownum
FROM
	(
	 SELECT
			@rownum := IF (@GROUP = city ,@rownum + 1, 1) AS r,
		  	@GROUP := city AS g,
			t1.*
	 FROM
		 (
			SELECT
				user_id,
				sex,
				city,
				amount
			FROM
				test
			GROUP BY
				user_id
			ORDER BY
				city,
				amount DESC
		) t1
	) t2
WHERE
	r < 4;