[英]SQL JOIN with WHERE condition when two rows' values are the same and one row matches to two different rows
我有这些表:
CREATE TABLE students(
id int NOT NULL PRIMARY KEY,
name VARCHAR(30) NOT NULL
);
CREATE TABLE studentsActivities(
studentId int NOT NULL,
activity VARCHAR(30) NOT NULL,
PRIMARY KEY (studentId, activity),
foreign KEY (studentId) REFERENCES students(id)
);
我必须返回所有打网球或足球的学生姓名。 但是,有一个我无法通过的测试用例,它是这样说的:
同名学生。
我不知道测试用例的确切实现,但我怀疑是学生 A 叫 Carl 打网球,学生 B 也叫 Carl 打足球,Carl 出现两次的情况。 我如何查询该数据库以获得这样的结果? 我创建了演示库来尝试:
CREATE TABLE students(
id int NOT NULL PRIMARY KEY,
name VARCHAR(30) NOT NULL
);
CREATE TABLE studentsActivities(
studentId int NOT NULL,
activity VARCHAR(30) NOT NULL,
PRIMARY KEY (studentId, activity),
foreign KEY (studentId) REFERENCES students(id)
);
INSERT INTO students
VALUES
(1, "Jeremy"),
(2, "Hannah"),
(3, "Luke"),
(4, "Frank"),
(5, "Sue"),
(6, "Sue"),
(7, "Peter");
INSERT INTO studentsActivities
VALUES
(1, "Tennis"),
(1, "Football"),
(2, "Running"),
(3, "Tennis"),
(4, "Football"),
(5, "Football"),
(6, "Tennis");
SQL 小提琴让我们假设传递集是:
Jeremy
Luke
Frank
Sue
Sue
我已经尝试过这两个查询,但没有一个给出正确的答案。
--- 1
SELECT s.name
FROM students s
JOIN studentsActivities sa
ON sa.studentId = s.id
WHERE activity = "Tennis"
UNION
SELECT s.name
FROM students s
JOIN studentsActivities sa
ON sa.studentId = s.id
WHERE activity = "Football"
--- Returns Frank Jeremy Luke Sue (missing one Sue)
--- 2
SELECT s.name
FROM students s
JOIN studentsActivities sa
ON sa.studentId = s.id
WHERE activity = "Tennis"
OR activity = "Football"
ORDER BY s.name;
--- Returns Frank Jeremy Jeremy Luke Sue Sue (too much Jeremies)
您可以使用exists
:
select s.*
from students s
where exists (
select 1
from studentsActivities sa
where sa.studentId = s.id and sa.activity in ('Tennis', 'Football')
)
id | name -: | :----- 1 | Jeremy 3 | Luke 4 | Frank 5 | Sue 6 | Sue
加入表格,仅过滤包含您想要的活动的行并返回不同的行:
select distinct s.id, s.name
from students s inner join studentsActivities a
on a.studentId = s.id
where a.activity in ('Tennis', 'Football')
见演示。
结果:
| id | name |
| --- | ------ |
| 1 | Jeremy |
| 3 | Luke |
| 4 | Frank |
| 5 | Sue |
| 6 | Sue |
如果您只想要没有 id 的学生姓名:
select s.name
from students s inner join studentsActivities a
on a.studentId = s.id
where a.activity in ('Tennis', 'Football')
group by s.id, s.name
见演示。
结果:
| name |
| ------ |
| Jeremy |
| Luke |
| Frank |
| Sue |
| Sue |
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