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); 

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; 

正确的写法-2

正确的写法-3

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