MySQL 分组获取每组前N条记录
MySQL 5.x 推荐方式二，MySQL8+直接窗口函数即可
最近遇到一个需求，需要根据某个字段或者两个字段进行分组，然后获取每组前25条数据进行展示，网上普遍的方法前篇一律，但是普遍查询效率低，需要借助索引优化一部分，这里介绍一种新的查询方式，无索引也能快速返回。
模拟数据来源此篇博客：https://blog.csdn.net/u014056045/article/details/126707342
数据
直接使用以上博客的表结构和数据做一个测试
表结构和表数据sql脚本如下：
DROP TABLE IF EXISTS `electrics`;
CREATE TABLE `electrics` (
`eid` int(0) NOT NULL AUTO_INCREMENT COMMENT '电能id',
`fid` int(0) NULL DEFAULT NULL COMMENT '工厂id',
`departname` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL COMMENT '区域名',
`energy` decimal(10, 2) NULL DEFAULT NULL COMMENT '耗能',
`energytime` datetime(0) NULL DEFAULT NULL COMMENT '耗能月份',
`createtime` datetime(0) NULL DEFAULT NULL COMMENT '创建时间',
`status` int(0) NULL DEFAULT NULL COMMENT '状态',
`week` int(0) NULL DEFAULT 0 COMMENT '第几周数',
PRIMARY KEY (`eid`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 76 CHARACTER SET = utf8 COLLATE = utf8_general_ci COMMENT = '电能' ROW_FORMAT = Dynamic;
表数据
INSERT INTO `electrics` VALUES (1, 1, '中央空调', 1707.18, '2021-11-01 00:00:00', '2021-12-01 16:43:42', 1, 40);
INSERT INTO `electrics` VALUES (2, 1, '线装', 8873.79, '2021-11-01 00:00:00', '2021-12-01 16:46:10', 1, 40);
INSERT INTO `electrics` VALUES (3, 1, '自动化', 3232.41, '2021-11-01 00:00:00', '2021-12-01 17:23:50', 1, 40);INSERT INTO `electrics` VALUES (9, 1, '办公区', 992.01, '2021-11-01 00:00:00', '2021-12-07 11:47:57', 1, 40);
INSERT INTO `electrics` VALUES (10, 1, '空压机', 5121.84, '2021-11-01 00:00:00', '2021-12-07 11:48:18', 1, 40);
INSERT INTO `electrics` VALUES (11, 1, '电梯', 53.06, '2021-11-01 00:00:00', '2021-12-07 11:48:43', 1, 40);
INSERT INTO `electrics` VALUES (12, 1, '加工中心', 2302.69, '2021-11-01 00:00:00', '2021-12-07 11:49:37', 1, 40);
INSERT INTO `electrics` VALUES (13, 1, '自动化', 4304.69, '2021-11-01 00:00:00', '2021-12-07 11:50:18', 1, 41);
INSERT INTO `electrics` VALUES (14, 1, '线装', 7030.03, '2021-11-01 00:00:00', '2021-12-07 11:51:32', 1, 41);
INSERT INTO `electrics` VALUES (15, 1, '办公区', 1923.07, '2021-11-01 00:00:00', '2021-12-07 11:51:52', 1, 41);
INSERT INTO `electrics` VALUES (16, 1, '空压机', 5014.48, '2021-11-01 00:00:00', '2021-12-07 11:52:13', 1, 41);
INSERT INTO `electrics` VALUES (17, 1, '电梯', 59.98, '2021-11-01 00:00:00', '2021-12-07 11:53:21', 1, 41);
INSERT INTO `electrics` VALUES (18, 1, '中央空调', 230.70, '2021-11-01 00:00:00', '2021-12-07 11:53:43', 1, 41);
INSERT INTO `electrics` VALUES (19, 1, '加工中心', 2749.01, '2021-11-01 00:00:00', '2021-12-07 11:54:01', 1, 41);
INSERT INTO `electrics` VALUES (20, 1, '自动化', 2063.53, '2021-11-01 00:00:00', '2021-12-07 11:54:42', 1, 42);
INSERT INTO `electrics` VALUES (21, 1, '线装', 6151.53, '2021-11-01 00:00:00', '2021-12-07 11:54:59', 1, 42);
INSERT INTO `electrics` VALUES (22, 1, '办公区', 1016.62, '2021-11-01 00:00:00', '2021-12-07 11:55:22', 1, 42);
INSERT INTO `electrics` VALUES (23, 1, '空压机', 4875.76, '2021-11-01 00:00:00', '2021-12-07 11:55:59', 1, 42);
INSERT INTO `electrics` VALUES (24, 1, '电梯', 59.98, '2021-11-01 00:00:00', '2021-12-07 11:56:19', 1, 42);
INSERT INTO `electrics` VALUES (25, 1, '中央空调', 92.28, '2021-11-01 00:00:00', '2021-12-07 11:56:40', 1, 42);
INSERT INTO `electrics` VALUES (26, 1, '加工中心', 1857.44, '2021-11-01 00:00:00', '2021-12-07 11:57:02', 1, 42);
INSERT INTO `electrics` VALUES (27, 1, '自动化', 2045.84, '2021-11-01 00:00:00', '2021-12-07 11:57:27', 1, 43);
INSERT INTO `electrics` VALUES (28, 1, '线装', 8853.80, '2021-11-01 00:00:00', '2021-12-07 11:57:47', 1, 43);
INSERT INTO `electrics` VALUES (29, 1, '办公区', 1045.84, '2021-11-01 00:00:00', '2021-12-07 11:58:16', 1, 43);
INSERT INTO `electrics` VALUES (30, 1, '空压机', 4814.24, '2021-11-01 00:00:00', '2021-12-07 11:58:39', 1, 43);
INSERT INTO `electrics` VALUES (31, 1, '电梯', 62.29, '2021-11-01 00:00:00', '2021-12-07 11:58:58', 1, 43);
INSERT INTO `electrics` VALUES (32, 1, '中央空调', 138.42, '2021-11-01 00:00:00', '2021-12-07 11:59:18', 1, 43);
INSERT INTO `electrics` VALUES (33, 1, '加工中心', 1766.69, '2021-11-01 00:00:00', '2021-12-07 11:59:37', 1, 43);
INSERT INTO `electrics` VALUES (34, 1, '自动化', 1932.80, '2021-11-01 00:00:00', '2021-12-07 11:59:53', 1, 44);
INSERT INTO `electrics` VALUES (35, 1, '线装', 9201.39, '2021-11-01 00:00:00', '2021-12-07 12:00:09', 1, 44);
INSERT INTO `electrics` VALUES (36, 1, '办公区', 1088.14, '2021-11-01 00:00:00', '2021-12-07 12:00:28', 1, 44);
INSERT INTO `electrics` VALUES (37, 1, '空压机', 5921.60, '2021-11-01 00:00:00', '2021-12-07 12:00:53', 1, 44);
INSERT INTO `electrics` VALUES (38, 1, '电梯', 46.14, '2021-11-01 00:00:00', '2021-12-07 12:03:51', 1, 44);
INSERT INTO `electrics` VALUES (39, 1, '中央空调', 322.98, '2021-11-01 00:00:00', '2021-12-07 12:04:14', 1, 44);
INSERT INTO `electrics` VALUES (40, 1, '加工中心', 2039.69, '2021-11-01 00:00:00', '2021-12-07 12:04:35', 1, 44);
INSERT INTO `electrics` VALUES (41, 1, '中央空调', 1807.18, '2022-02-28 00:00:00', '2022-03-31 16:57:48', 1, 9);
INSERT INTO `electrics` VALUES (42, 1, '线装', 8763.79, '2022-02-28 00:00:00', '2022-03-31 16:57:48', 1, 9);
INSERT INTO `electrics` VALUES (43, 1, '自动化', 3232.41, '2022-02-28 00:00:00', '2022-03-31 16:57:48', 1, 9);
INSERT INTO `electrics` VALUES (44, 1, '办公区', 992.01, '2022-02-28 00:00:00', '2022-03-31 16:57:48', 1, 9);
INSERT INTO `electrics` VALUES (45, 1, '空压机', 5121.84, '2022-02-28 00:00:00', '2022-03-31 16:57:48', 1, 9);
INSERT INTO `electrics` VALUES (46, 1, '电梯', 51.06, '2022-02-28 00:00:00', '2022-03-31 16:57:48', 1, 9);
INSERT INTO `electrics` VALUES (47, 1, '加工中心', 2002.69, '2022-02-28 00:00:00', '2022-03-31 16:57:48', 1, 9);
INSERT INTO `electrics` VALUES (48, 1, '中央空调', 1707.18, '2022-03-01 00:00:00', '2022-03-31 16:59:17', 1, 10);
INSERT INTO `electrics` VALUES (49, 1, '线装', 8873.79, '2022-03-01 00:00:00', '2022-03-31 16:59:17', 1, 10);
INSERT INTO `electrics` VALUES (50, 1, '自动化', 3132.41, '2022-03-01 00:00:00', '2022-03-31 16:59:17', 1, 10);
INSERT INTO `electrics` VALUES (51, 1, '办公区', 992.01, '2022-03-01 00:00:00', '2022-03-31 16:59:17', 1, 10);
INSERT INTO `electrics` VALUES (52, 1, '空压机', 5121.84, '2022-03-01 00:00:00', '2022-03-31 16:59:17', 1, 10);
INSERT INTO `electrics` VALUES (53, 1, '电梯', 50.06, '2022-03-01 00:00:00', '2022-03-31 16:59:17', 1, 10);
INSERT INTO `electrics` VALUES (54, 1, '加工中心', 2302.69, '2022-03-01 00:00:00', '2022-03-31 16:59:17', 1, 10);
INSERT INTO `electrics` VALUES (55, 1, '中央空调', 1207.18, '2022-03-14 00:00:00', '2022-03-31 16:59:34', 1, 11);
INSERT INTO `electrics` VALUES (56, 1, '线装', 8873.79, '2022-03-14 00:00:00', '2022-03-31 16:59:34', 1, 11);
INSERT INTO `electrics` VALUES (57, 1, '自动化', 3232.41, '2022-03-14 00:00:00', '2022-03-31 16:59:34', 1, 11);
INSERT INTO `electrics` VALUES (58, 1, '办公区', 982.01, '2022-03-14 00:00:00', '2022-03-31 16:59:34', 1, 11);
INSERT INTO `electrics` VALUES (59, 1, '空压机', 5121.84, '2022-03-14 00:00:00', '2022-03-31 16:59:34', 1, 11);
INSERT INTO `electrics` VALUES (60, 1, '电梯', 53.06, '2022-03-14 00:00:00', '2022-03-31 16:59:34', 1, 11);
INSERT INTO `electrics` VALUES (61, 1, '加工中心', 2202.69, '2022-03-14 00:00:00', '2022-03-31 16:59:34', 1, 11);
INSERT INTO `electrics` VALUES (62, 1, '中央空调', 1707.18, '2022-03-21 00:00:00', '2022-03-31 16:59:46', 1, 12);
INSERT INTO `electrics` VALUES (63, 1, '线装', 8873.79, '2022-03-21 00:00:00', '2022-03-31 16:59:46', 1, 12);
INSERT INTO `electrics` VALUES (64, 1, '自动化', 3232.41, '2022-03-21 00:00:00', '2022-03-31 16:59:46', 1, 12);
INSERT INTO `electrics` VALUES (65, 1, '办公区', 992.01, '2022-03-21 00:00:00', '2022-03-31 16:59:46', 1, 12);
INSERT INTO `electrics` VALUES (66, 1, '空压机', 5021.84, '2022-03-21 00:00:00', '2022-03-31 16:59:46', 1, 12);
INSERT INTO `electrics` VALUES (67, 1, '电梯', 53.06, '2022-03-21 00:00:00', '2022-03-31 16:59:46', 1, 12);
INSERT INTO `electrics` VALUES (68, 1, '加工中心', 2102.69, '2022-03-21 00:00:00', '2022-03-31 16:59:46', 1, 12);
INSERT INTO `electrics` VALUES (69, 1, '中央空调', 1707.18, '2022-03-28 00:00:00', '2022-03-31 16:59:56', 1, 13);
INSERT INTO `electrics` VALUES (70, 1, '线装', 8873.79, '2022-03-28 00:00:00', '2022-03-31 16:59:56', 1, 13);
INSERT INTO `electrics` VALUES (71, 1, '自动化', 3232.41, '2022-03-28 00:00:00', '2022-03-31 16:59:56', 1, 13);
INSERT INTO `electrics` VALUES (72, 1, '办公区', 992.01, '2022-03-28 00:00:00', '2022-03-31 16:59:56', 1, 13);
INSERT INTO `electrics` VALUES (73, 1, '空压机', 5121.84, '2022-03-28 00:00:00', '2022-03-31 16:59:56', 1, 13);
INSERT INTO `electrics` VALUES (74, 1, '电梯', 63.06, '2022-03-28 00:00:00', '2022-03-31 16:59:56', 1, 13);
INSERT INTO `electrics` VALUES (75, 1, '加工中心', 2502.69, '2022-03-28 00:00:00', '2022-03-31 16:59:56', 1, 13);
方式一
这个方式就是上一篇博客里介绍的方式，此种方式，数据少时，查询效率还行，就如同模拟数据，但是当数据量大一点，比如400条左右时（这也“太多了”点。。。），查询就会变得效率很低
sql 书写方式1
SELECT
*
FROM
electrics e
WHERE WEEK <> 0
AND 5 > (SELECT count( 1 ) FROM electrics el WHERE el.departname = e.departname AND el.energytime > e.energytime)
ORDER BY departname,week;
sql 书写方式2
SELECT
*
FROM
electrics e
WHERE WEEK <> 0
AND EXISTS (SELECT count(1) FROM electrics el WHERE el.departname = e.departname AND el.energytime > e.energytime HAVING COUNT(1) < 5)
ORDER BY departname,week;
方式二（推荐）
使用模拟 ROW_NUMBER + PARTITION 的方式，这种查询效率，在无索引的情况下，相比上方的查询sql效率也高一些，书写方式如下：
SELECT
*
FROM (
SELECT
CASE
WHEN @cn != departname THEN @rownum := 1
ELSE @rownum := @rownum + 1
END AS no,
@cn := departname AS departname,
eid,
energy,
energytime,
createtime,
status,
week
FROM
(SELECT @rownum := 0) r,
(SELECT @cn := ' ') p,
electrics
ORDER BY departname,energytime DESC
) sub
WHERE no <= 5;
方式三 MySQL8+
使用数据库提供的窗口函数ROW_NUMBER()即可，数据库版本MySQL8+，不用考虑，直接用
SELECT * FROM (
SELECT
ROW_NUMBER() OVER (PARTITION BY departname ORDER BY energytime DESC) AS rwnum,
departname,
eid,
energy,
energytime,
createtime,
status,
week
FROM
electrics
) sub
WHERE rwnum <= 5;
总结
线上单表数据不大，目前也才几千条，没索引的情况下，使用方式一，12s，使用方式二，0.05s，高下立见，但是网上普遍都是方式一，感兴趣的可以试试方式二。
数据库版本MySQL8+的就不用考虑，直接用窗口函数即可。