MySQL查询列必须和groupby字段一致吗?
目录 场景:查询各部门薪水最高的员工。 MySQL group by是如何决定哪一条数据留下的? 那么target list和group by column不匹配就一定不能执行吗?
MySQL版本:8.0.27 场景:查询各部门薪水最高的员工。CREATE TABLE `employee` ( `id` int NOT NULL AUTO_INCREMENT COMMENT "主键ID", `dept` int NOT NULL COMMENT "部门", `user` varchar(32) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL COMMENT "员工", `salary` int NULL DEFAULT NULL COMMENT "薪水", `is_deleted` tinyint(1) NOT NULL DEFAULT 0 COMMENT "是否删除", `remark` varchar(512) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL COMMENT "备注", `modify_time` datetime(3) NOT NULL DEFAULT CURRENT_TIMESTAMP(3) ON UPDATE CURRENT_TIMESTAMP(3) COMMENT "修改时间", PRIMARY KEY (`id`) USING BTREE ) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci COMMENT = "员工" ROW_FORMAT = Dynamic; INSERT INTO `employee` VALUES (1, 1, "张三", 1000, 0, NULL, "2021-12-23 09:20:19.606"); INSERT INTO `employee` VALUES (2, 1, "李四", 1500, 0, NULL, "2021-12-23 09:20:21.679"); INSERT INTO `employee` VALUES (3, 1, "王五", 2000, 0, NULL, "2021-12-23 09:20:23.371"); INSERT INTO `employee` VALUES (4, 2, "赵六", 1000, 0, NULL, "2021-12-23 09:21:59.373"); INSERT INTO `employee` VALUES (5, 2, "孙七", 1500, 0, NULL, "2021-12-23 09:22:15.000");
SELECT * FROM employee ;
方法一:SELECT t1.* FROM employee t1 LEFT JOIN employee t2 ON t2.dept = t1.dept AND t1.salary < t2.salary WHERE t2.salary IS NULL;
方法二:SELECT * FROM ( SELECT * FROM `employee` ORDER BY dept, salary DESC LIMIT 1000 ) t GROUP BY dept;
(不加limit可能会失效)
看起来结果是一样的,但第二种其实是会有问题的。 MySQL group by是如何决定哪一条数据留下的?
MySQL通过sql_mode来提供SQL语句的合法性检查,
在默认情况下,MySQL允许查询列target list中出现除了group by column、聚集函数等以外的表达式。
但是,那些不参与group by的字段具体会返回哪条数据的值在MySQL中是处于未定义规则的状态,
MySQL不承诺一定会返回哪条数据。 分组前的数据:
SELECT * FROM employee ORDER BY dept, salary DESC LIMIT 1000;
看起来方法二返回的是每个分组中的第一条的数据,
但实际上还会与存储引擎、物理位置、索引等有关,
如果是InnoDB的话,取决于在B+Tree上命中的第一条索引,
这里不展开说明,毕竟不是安全的用法,
有的时候可能返回的结果并不是我们想要的。
关于B+Tree,可以看下这篇文章:
通过B+Tree平衡多叉树理解InnoDB引擎的聚集和非聚集索引
所以对于target list中出现的不明确的列,MySQL是不确定哪一条数据留下的。
对于语法限制比较严格的数据库,都不支持target list中出现语义不明确的列,
MySQL中提供了一个修正的sql_mode,ONLY_FULL_GROUP_BY。 SET SESSION sql_mode = "ONLY_FULL_GROUP_BY";
再执行方法二的SQL就被拒绝了: SELECT * FROM ( SELECT * FROM `employee` ORDER BY dept, salary DESC LIMIT 1000 ) t GROUP BY dept > 1055 - Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column "t.id" which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by > 时间: 0s
"only_full_group_by"模式下MySQL会对target list和group by column中的基础列、表达式、别名列进行严格匹配。 那么target list和group by column不匹配就一定不能执行吗?
我们看下另外一条SQL: # 订单 CREATE TABLE `order` ( `order_id` int NOT NULL AUTO_INCREMENT COMMENT "订单ID", `order_amount` int NULL DEFAULT NULL COMMENT "订单金额", PRIMARY KEY (`order_id`) USING BTREE ) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci COMMENT = "订单" ROW_FORMAT = DYNAMIC; INSERT INTO `order` VALUES (1, 100); INSERT INTO `order` VALUES (2, 103); INSERT INTO `order` VALUES (3, 100); # 订单明细 CREATE TABLE `order_detail` ( `order_detail_id` int NOT NULL AUTO_INCREMENT COMMENT "主键ID", `order_id` int NOT NULL COMMENT "订单ID", `goods` varchar(32) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL COMMENT "商品名称", `goods_amount` int NOT NULL COMMENT "商品金额", PRIMARY KEY (`order_detail_id`) USING BTREE ) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci COMMENT = "订单明细" ROW_FORMAT = DYNAMIC; INSERT INTO `order_detail` VALUES (1, 1, "苹果", 10); INSERT INTO `order_detail` VALUES (2, 1, "橙子", 20); INSERT INTO `order_detail` VALUES (3, 1, "香蕉", 70); INSERT INTO `order_detail` VALUES (4, 2, "橘子", 50); INSERT INTO `order_detail` VALUES (5, 2, "菠萝", 53);
查询订单中所有商品 SELECT t1.order_id, t1.order_amount, GROUP_CONCAT( t2.goods, t2.goods_amount ) FROM `order` t1 LEFT JOIN order_detail t2 ON t2.order_id = t1.order_id GROUP BY t1.order_id;
这条SQL的target list和group by column并不是严格匹配的,但是也可以执行,
注意
t1.order_id是订单表的主键。
所以在"only_full_group_by"模式下,如果MySQL可以确定target list中所有列的返回值,
那么,即使target list和group by column中的基础列、表达式、别名列等不严格匹配,
MySQL也会认为它的语义是明确的,因此该条语句可以顺利通过。
原文 http://www.cnblogs.com/captaincat/p/15723369.html