考虑经典的学生和类(class)多对多关系,其中一个学生可以参加多个类(class),并且一个类(class)包含多个学生。
CREATE TABLE students(
id serial PRIMARY KEY,
name text,
gender text NOT NULL
);
CREATE TABLE schools(
id serial PRIMARY KEY,
name text,
);
CREATE TABLE classes(
id serial PRIMARY KEY,
name text,
school_id integer NOT NULL REFERENCES schools (id)
);
CREATE TABLE students_classes(
id serial PRIMARY KEY,
class_id integer NOT NULL REFERENCES classes (id),
student_id integer NOT NULL REFERENCES students (id),
);
整体查询要大得多 - 考虑到学校和其他因素会增加问题的复杂性。所以我需要使用窗口函数来获取诸如 total_students
这样的东西。
我想要一个查询来获取所有类(class)、该类(class)注册的学生总数、注册的男生人数和女生人数。
class_id | n_students | n_guys | n_girls
____________________________________________
| | |
到目前为止我有以下信息,我可以帮忙了解一下男孩和女孩的数量吗?
SELECT
school_id,
w.class_id,
w.n_students,
w.n_guys,
w.n_girls
FROM schools
JOIN classes ON classes.school_id = schools.id
JOIN (
c.id AS class_id,
COUNT(*) OVER (PARTITION BY sc.class_id) AS n_students,
{Something} AS n_guys,
{Something} AS n_girls
FROM students_classes AS sc
JOIN classes AS c ON sc.class_id = c.id
) as w ON w.class_id = classes.id
WHERE school_id = 81;
最佳答案
你可以使用这个,不需要使用windows/analytic函数
将 male
和 female
更改为 students.gender
列的文本值
SELECT
s.school_id,
c.class_id,
COUNT(*) AS n_students,
SUM(CASE WHEN st.gender = 'male' THEN 1 ELSE 0 END) AS n_guys,
SUM(CASE WHEN st.gender = 'female' THEN 1 ELSE 0 END) AS n_girls
FROM schools s
INNER JOIN classes c
ON c.school_id = schools.id
INNER JOIN students_classes sc
ON sc.class_id = classes.id
INNER JOIN students st
ON st.id = sc.student_id
WHERE s.school_id = 81
GROUP BY s.school_id, c.class_id
ORDER BY s.school_id, c.class_id;
关于sql - 结合窗口函数和条件,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/52195287/