MySQL 发现一个问题,我无法理解

drop table if exists tmp_a; CREATE TABLE `tmp_a` ( `date_a` date DEFAULT NULL, `id` int(11) DEFAULT NULL, UNIQUE KEY `id_idx` (`id`,`date_a`) USING BTREE ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4; drop table if exists tmp_b; CREATE TABLE `tmp_b` ( `date_b` date DEFAULT NULL, `id` int(11) EFAULT NULL, UNIQUE KEY `id_idx` (`id`,`date_b`) USING BTREE ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4; truncate TABLE grab.tmp_a; truncate TABLE grab.tmp_b; insert into grab.tmp_a(id,date_a)values(1,'2019-01-01'),(1,'2019-01-02'); insert into grab.tmp_b(id,date_b)values(1,'2019-01-01'),(1,'2019-01-02'); 

我运行

SELECT * from grab.tmp_a as a inner join grab.tmp_b as b on a.id = b.id WHERE a.date_a > b.date_b; 

结果会有一条数据出来