union all

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

id	device_id	gender	age	university	gpa	active_days_within_30	question_cnt	answer_cnt
1	2138	male	21	北京大学	3.4	7	2	12
2	3214	male		复旦大学	4	15	5	25
3	6543	female	20	北京大学	3.2	12	3	30
4	2315	female	23	浙江大学	3.6	5	1	2
5	5432	male	25	山东大学	3.8	20	15	70
6	2131	male	28	山东大学	3.3	15	7	13
7	4321	male	26	复旦大学	3.6	9	6	52

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）：

device_id	gender	age	gpa
5432	male	25	3.8
2131	male	28	3.3
2138	male	21	3.4
3214	male	None	4
5432	male	25	3.8
2131	male	28	3.3
4321	male	28	3.6

示例

输入：

drop table if exists `user_profile`;
drop table if  exists `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');

输出：

5432|male|25|3.8
2131|male|28|3.3
2138|male|21|3.4
3214|male|None|4.0
5432|male|25|3.8
2131|male|28|3.3
4321|male|28|3.6

解题

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

union all

select 
    device_id, gender, age, gpa
from user_profile
where gender='male'

题解

NION和UNION ALL都是用来合并两个或多个SELECT语句的结果集的。

不同之处在于UNION会自动去重，而UNION ALL不会。

以下是使用UNION的示例：

假设有两个表：

Table1
+----+--------+
| ID |  Name  |
+----+--------+
|  1 | Alice  |
|  2 | Bob    |
|  3 | Charlie|
+----+--------+

Table2
+----+------+
| ID | Name |
+----+------+
|  3 | Dave |
|  4 | Eve  |
|  5 | Frank|
+----+------+

我们可以使用以下查询语句来将表1和表2中的数据合并：

SELECT Name FROM Table1
UNION
SELECT Name FROM Table2;

结果如下：

+--------+
|  Name  |
+--------+
| Alice  |
| Bob    |
| Charlie|
| Dave   |
| Eve    |
| Frank  |
+--------+

可以看到，结果中的重复行"Dave"被自动去除了。

下面是使用UNION ALL的示例：

SELECT Name FROM Table1
UNION ALL
SELECT Name FROM Table2;

结果如下：

+--------+
|  Name  |
+--------+
| Alice  |
| Bob    |
| Charlie|
| Dave   |
| Eve    |
| Frank  |
| Charlie|
| Dave   |
| Eve    |
| Frank  |
+--------+

可以看到，结果中出现了重复行"Dav"和"Eve"，因为我们使用了UNION ALL而不是UNION。