MySQL去重查询只保留一条最新的记录

需求：MySQL去重查询只保留一条最新的记录

---

易错的写法

表结构与初始数据如下SQL文件：

-- MySQL dump 10.13  Distrib 8.0.28, for Linux (x86_64)
--
-- Host: localhost    Database: test01
-- ------------------------------------------------------
-- Server version	8.0.28-0ubuntu0.20.04.3

/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!50503 SET NAMES utf8 */;
/*!40103 SET @OLD_TIME_ZONE=@@TIME_ZONE */;
/*!40103 SET TIME_ZONE='+00:00' */;
/*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */;
/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;
/*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */;
/*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */;

--
-- Table structure for table `MyClass`
--

DROP TABLE IF EXISTS `MyClass`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!50503 SET character_set_client = utf8mb4 */;
CREATE TABLE `MyClass` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` char(20) NOT NULL,
  `sex` int NOT NULL DEFAULT '0',
  `degree` double(16,2) DEFAULT NULL,
  `inserttime` text,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
/*!40101 SET character_set_client = @saved_cs_client */;

--
-- Dumping data for table `MyClass`
--

LOCK TABLES `MyClass` WRITE;
/*!40000 ALTER TABLE `MyClass` DISABLE KEYS */;
INSERT INTO `MyClass` VALUES (1,'Tom',0,96.45,'2022-02-19 18:00:00'),(2,'Joan',0,82.99,'2022-02-19 18:20:00'),(3,'Wang',0,96.59,'2022-02-19 18:30:00'),(4,'Tom',0,96.45,'2022-02-19 17:00:00'),(5,'Joan',0,82.99,'2022-02-19 18:25:00'),(6,'Wang',0,96.59,'2022-02-19 18:35:00');
/*!40000 ALTER TABLE `MyClass` ENABLE KEYS */;
UNLOCK TABLES;
/*!40103 SET TIME_ZONE=@OLD_TIME_ZONE */;

/*!40101 SET SQL_MODE=@OLD_SQL_MODE */;
/*!40014 SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS */;
/*!40014 SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS */;
/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
/*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */;
/*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */;
/*!40111 SET SQL_NOTES=@OLD_SQL_NOTES */;

-- Dump completed on 2022-02-20  4:35:15

SQL统计语句：

#全表扫描查询
SELECT * FROM test01.MyClass;
#根据'name'分组统计查询最新一条数据
SELECT * FROM test01.MyClass
WHERE 
	name in (SELECT name FROM test01.MyClass GROUP BY name)
	AND inserttime in (SELECT max(inserttime) FROM test01.MyClass GROUP BY name);

全表扫描查询_结果：

根据’name’字段去重统计查询最新数据_结果：

---

正确的写法-1

--------------------------------------------错误纠正UPDATE Sun May 1 00:28:18 CST 2022

初始数据如下：

[{"id":1, "name":"Tom", "sex":0, "degree":96.45, "inserttime":"2022-02-19 18:00:00"},
 {"id":2, "name":"Joan", "sex":0, "degree":82.99, "inserttime":"2022-02-19 18:20:00"},
 {"id":3, "name":"Wang", "sex":0, "degree":96.59, "inserttime":"2022-02-19 18:00:00"},
 {"id":4, "name":"Tom", "sex":0, "degree":96.45, "inserttime":"2022-02-19 17:00:00"},
 {"id":5, "name":"Joan", "sex":0, "degree":82.99, "inserttime":"2022-02-19 18:00:00"},
 {"id":6, "name":"Wang", "sex":0, "degree":96.59, "inserttime":"2022-02-19 18:35:00"},
 {"id":7, "name":"Wang", "sex":0, "degree":90.0, "inserttime":"2022-02-19 18:36:00"}]

#全表扫描查询
SELECT 
    *
FROM
    test01.MyClass
ORDER BY inserttime DESC;
    
#根据'name'字段去重统计查询最新数据(错误的写法)
#SELECT * FROM test01.MyClass
#WHERE 
#	name in (SELECT name FROM test01.MyClass GROUP BY name)
#	AND inserttime in (SELECT max(inserttime) FROM test01.MyClass GROUP BY name);

#去重查询只保留一条最新的记录(正确的写法-1)
SELECT 
    classtab.*
FROM
    (SELECT 
        name, MAX(inserttime) inserttime
    FROM
        test01.MyClass
    GROUP BY name) tmp
        LEFT JOIN
    test01.MyClass classtab ON classtab.name = tmp.name
        AND classtab.inserttime = tmp.inserttime;

#全表扫描查询_结果:

#去重查询只保留一条最新的记录(正确的写法-1)_结果:

正确的写法-2

正确的写法-3

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