第二张子表存在记录时按父表字段汇总子表字段的SQL写法

现有三张业务表，结构及测试数据如下：

• tournament_oc（赛事主表）

• match_oc（比赛明细表）

• stat_oc（赛事统计表）

需求 #

筛选存在至少1条关联match_oc记录的赛事，按赛事ID分组，统计对应stat_oc.value_的总和，预期输出：

错误写法与原因 #

最初尝试的SQL如下：

SELECT
    t.id_ AS tournament_oc_id, sum(s.value_) AS sum_value
FROM
    tournament_oc AS t
        JOIN
    match_oc AS m ON m.tournament_oc_id = t.id_
        JOIN
    stat_oc as s on s.tournament_oc_id = t.id_
GROUP BY t.id_

执行后返回错误结果sum_value=4，根因是tournament_oc.id_=1关联2条match_oc记录，JOIN后产生笛卡尔积，导致stat_oc的记录被重复复制，求和时数值翻倍。

核心思路是避免多表一对多关联产生的行放大问题，两种常用实现：

写法1：EXISTS判断（推荐，性能最优） #

直接用EXISTS子查询判断赛事是否存在关联比赛，不需要关联match表全量数据，避免笛卡尔积：

SELECT
    t.id_ AS tournament_oc_id,
    SUM(s.value_) AS sum_value
FROM tournament_oc t
JOIN stat_oc s ON s.tournament_oc_id = t.id_
WHERE EXISTS (
    SELECT 1 
    FROM match_oc m 
    WHERE m.tournament_oc_id = t.id_
)
GROUP BY t.id_;

写法2：match表预去重后关联 #

如果偏好JOIN写法，先将match表按赛事ID去重，保证每个赛事只保留1条匹配记录，再关联stat表求和：

SELECT
    t.id_ AS tournament_oc_id,
    SUM(s.value_) AS sum_value
FROM tournament_oc t
JOIN (
    SELECT DISTINCT tournament_oc_id 
    FROM match_oc
) m ON m.tournament_oc_id = t.id_
JOIN stat_oc s ON s.tournament_oc_id = t.id_
GROUP BY t.id_;

两种写法执行结果均符合预期：仅返回存在关联比赛的赛事ID=1，求和值为2。

附：复现测试环境的SQL脚本

CREATE DATABASE  IF NOT EXISTS `test` /*!40100 DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci */ /*!80016 DEFAULT ENCRYPTION='N' */;
USE `test`;

/*!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 */;

DROP TABLE IF EXISTS `match_oc`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!50503 SET character_set_client = utf8mb4 */;
CREATE TABLE `match_oc` (
  `id_` int NOT NULL AUTO_INCREMENT,
  `tournament_oc_id` int NOT NULL,
  PRIMARY KEY (`id_`),
  KEY `fk__match_oc__tournament_oc_id_idx` (`tournament_oc_id`),
  CONSTRAINT `fk__match_oc__tournament_oc_id` FOREIGN KEY (`tournament_oc_id`) REFERENCES `tournament_oc` (`id_`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
/*!40101 SET character_set_client = @saved_cs_client */;

LOCK TABLES `match_oc` WRITE;
/*!40000 ALTER TABLE `match_oc` DISABLE KEYS */;
INSERT INTO `match_oc` VALUES (1,1),(2,1);
/*!40000 ALTER TABLE `match_oc` ENABLE KEYS */;
UNLOCK TABLES;

DROP TABLE IF EXISTS `stat_oc`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!50503 SET character_set_client = utf8mb4 */;
CREATE TABLE `stat_oc` (
  `id_` int NOT NULL AUTO_INCREMENT,
  `tournament_oc_id` int NOT NULL,
  `value_` tinyint DEFAULT NULL,
  PRIMARY KEY (`id_`),
  KEY `fk__stat_oc__tournament_oc_id_idx` (`tournament_oc_id`),
  CONSTRAINT `fk__stat_oc__tournament_oc_id` FOREIGN KEY (`tournament_oc_id`) REFERENCES `tournament_oc` (`id_`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
/*!40101 SET character_set_client = @saved_cs_client */;

LOCK TABLES `stat_oc` WRITE;
/*!40000 ALTER TABLE `stat_oc` DISABLE KEYS */;
INSERT INTO `stat_oc` VALUES (1,1,1),(2,1,1),(3,2,1);
/*!40000 ALTER TABLE `stat_oc` ENABLE KEYS */;
UNLOCK TABLES;

DROP TABLE IF EXISTS `tournament_oc`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!50503 SET character_set_client = utf8mb4 */;
CREATE TABLE `tournament_oc` (
  `id_` int NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id_`)
) ENGINE=InnoDB AUTO_INCREMENT=26193 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
/*!40101 SET character_set_client = @saved_cs_client */;

LOCK TABLES `tournament_oc` WRITE;
/*!40000 ALTER TABLE `tournament_oc` DISABLE KEYS */;
INSERT INTO `tournament_oc` VALUES (1),(2);
/*!40000 ALTER TABLE `tournament_oc` 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 */;

内容的提问来源于stack exchange，提问作者Jossy