SQL union ALL用法

union all 操作符用于合并两个或多个 SELECT语句的结果集，并且不做去重操作。

可能这样讲比较抽象，以一道题为例子：

描述

题目：现在运营想要分别查看学校为山东大学或者性别为男性的用户的device_id、gender、age和gpa数据，请取出相应结果，结果不去重。

示例：user_profile

根据示例，你的查询应返回以下结果（注意输出的顺序，先输出学校为山东大学再输出性别为男生的信息）：

示例1

输入：

drop table if exists `user_profile`;drop table ifexists `question_practice_detail`;CREATE TABLE `user_profile` (`id` int NOT NULL,`device_id` int NOT NULL,`gender` varchar(14) NOT NULL,`age` int ,`university` varchar(32) NOT NULL,`gpa` float,`active_days_within_30` int ,`question_cnt` int ,`answer_cnt` int );CREATE TABLE `question_practice_detail` (`id` int NOT NULL,`device_id` int NOT NULL,`question_id`int NOT NULL,`result` varchar(32) NOT NULL);CREATE TABLE `question_detail` (`id` int NOT NULL,`question_id`int NOT NULL,`difficult_level` varchar(32) NOT NULL);INSERT INTO user_profile VALUES(1,2138,'male',21,'北京大学',3.4,7,2,12);INSERT INTO user_profile VALUES(2,3214,'male',null,'复旦大学',4.0,15,5,25);INSERT INTO user_profile VALUES(3,6543,'female',20,'北京大学',3.2,12,3,30);INSERT INTO user_profile VALUES(4,2315,'female',23,'浙江大学',3.6,5,1,2);INSERT INTO user_profile VALUES(5,5432,'male',25,'山东大学',3.8,20,15,70);INSERT INTO user_profile VALUES(6,2131,'male',28,'山东大学',3.3,15,7,13);INSERT INTO user_profile VALUES(7,4321,'male',28,'复旦大学',3.6,9,6,52);INSERT INTO question_practice_detail VALUES(1,2138,111,'wrong');INSERT INTO question_practice_detail VALUES(2,3214,112,'wrong');INSERT INTO question_practice_detail VALUES(3,3214,113,'wrong');INSERT INTO question_practice_detail VALUES(4,6543,111,'right');INSERT INTO question_practice_detail VALUES(5,2315,115,'right');INSERT INTO question_practice_detail VALUES(6,2315,116,'right');INSERT INTO question_practice_detail VALUES(7,2315,117,'wrong');INSERT INTO question_practice_detail VALUES(8,5432,117,'wrong');INSERT INTO question_practice_detail VALUES(9,5432,112,'wrong');INSERT INTO question_practice_detail VALUES(10,2131,113,'right');INSERT INTO question_practice_detail VALUES(11,5432,113,'wrong');INSERT INTO question_practice_detail VALUES(12,2315,115,'right');INSERT INTO question_practice_detail VALUES(13,2315,116,'right');INSERT INTO question_practice_detail VALUES(14,2315,117,'wrong');INSERT INTO question_practice_detail VALUES(15,5432,117,'wrong');INSERT INTO question_practice_detail VALUES(16,5432,112,'wrong');INSERT INTO question_practice_detail VALUES(17,2131,113,'right');INSERT INTO question_practice_detail VALUES(18,5432,113,'wrong');INSERT INTO question_practice_detail VALUES(19,2315,117,'wrong');INSERT INTO question_practice_detail VALUES(20,5432,117,'wrong');INSERT INTO question_practice_detail VALUES(21,5432,112,'wrong');INSERT INTO question_practice_detail VALUES(22,2131,113,'right');INSERT INTO question_practice_detail VALUES(23,5432,113,'wrong');INSERT INTO question_detail VALUES(1,111,'hard');INSERT INTO question_detail VALUES(2,112,'medium');INSERT INTO question_detail VALUES(3,113,'easy');INSERT INTO question_detail VALUES(4,115,'easy');INSERT INTO question_detail VALUES(5,116,'medium');INSERT INTO question_detail VALUES(6,117,'easy');

复制输出：

5432|male|25|3.82131|male|28|3.32138|male|21|3.43214|male|None|4.05432|male|25|3.82131|male|28|3.34321|male|28|3.6

解题思路 一：

但总会存在一个人满足了两个条件只筛选一次。这里的坑时使用or，因为or自带去重，而union等价于or，但union all 可以不去重，所以本体考察or与union的细节使用

SELECT distinct up.device_id,up.gender,up.age,up.gpa,up.university FROM user_profile upLEFT JOIN question_practice_detail qd ON up.device_id= qd.device_idWHERE up.university = "山东大学" OR up.gender = "male"ORDER BY up.university desc,up.gender desc

上面方法错误：

解题思路 二：

使用UNION ALL 把两个表链接起来 利用UNION ALL去方式粘贴两个表完成题目中的默认排序，并且题目中提到的可以重复，则不去重

select device_id , gender , age , gpa from user_profile where university = '山东大学' UNION ALLselect device_id , gender , age , gpa from user_profile where gender = 'male'