题目

在有成绩的学生中，统计选修至少两门课的人数百分比。

知识点：子查询、条件求和、比率

运行环境：本站提供在线 SQL 运行环境。

思路

分母是有成绩的学生人数，分子是其中选了至少两门的人数。先按人算出门数，再在外层做条件求和除以人数。

乘 100.0 得到百分比。邱石没有成绩，不进分母。

解法

-- 第93题：统计至少选修两门课的学生占比
-- 样例数据：学生选课 + 员工部门。最后一条 SELECT 为本题查询。

DROP TABLE IF EXISTS score;
DROP TABLE IF EXISTS course;
DROP TABLE IF EXISTS student;
DROP TABLE IF EXISTS teacher;
DROP TABLE IF EXISTS emp;
DROP TABLE IF EXISTS dept;

CREATE TABLE teacher (
  tid TEXT PRIMARY KEY,
  tname TEXT NOT NULL
);

CREATE TABLE student (
  sid TEXT PRIMARY KEY,
  sname TEXT NOT NULL,
  ssex TEXT,
  birthday TEXT,
  city TEXT,
  email TEXT
);

CREATE TABLE course (
  cid TEXT PRIMARY KEY,
  cname TEXT NOT NULL,
  tid TEXT,
  credit INTEGER
);

CREATE TABLE score (
  sid TEXT,
  cid TEXT,
  score REAL,
  exam_on TEXT
);

CREATE TABLE dept (
  did TEXT PRIMARY KEY,
  dname TEXT NOT NULL
);

CREATE TABLE emp (
  eid TEXT PRIMARY KEY,
  ename TEXT NOT NULL,
  did TEXT,
  manager_id TEXT,
  salary INTEGER,
  hired_on TEXT
);

INSERT INTO teacher(tid, tname) VALUES
('T01', '何启明'),
('T02', '陆知秋'),
('T03', '裴南'),
('T04', '江衡');

INSERT INTO student(sid, sname, ssex, birthday, city, email) VALUES
('S01', '林晓', '女', '2003-03-12', '杭州', 'linxiao@campus.test'),
('S02', '周衡', '男', '2002-11-08', '宁波', 'zhouheng@campus.test'),
('S03', '沈予', '女', '2003-06-21', '杭州', 'shenyu@campus.test'),
('S04', '韩牧', '男', '2002-07-09', '温州', 'hanmu@campus.test'),
('S05', '叶安', '女', '2004-01-30', '杭州', 'yean@campus.test'),
('S06', '方迟', '男', '2001-12-19', '嘉兴', 'fangchi@campus.test'),
('S07', '苏晚', '女', '2003-09-04', '宁波', 'suwan@campus.test'),
('S08', '程远', '男', '2004-05-17', '杭州', 'chengyuan@campus.test'),
('S09', '顾星', '女', '2002-10-28', '绍兴', 'guxing@campus.test'),
('S10', '梁野', '男', '2001-02-08', '温州', 'liangye@campus.test'),
('S11', '夏禾', '女', '2003-08-16', '杭州', 'xiahe@campus.test'),
('S12', '许川', '男', '2002-04-25', '宁波', 'xuchuan@campus.test'),
('S13', '丁南', '女', '2004-04-18', '杭州', 'dingnan@campus.test'),
('S14', '蔡河', '男', '2001-06-03', '嘉兴', 'caihe@campus.test'),
('S15', '闻青', '女', '2002-08-22', '绍兴', 'wenqing@campus.test'),
('S16', '邱石', '男', '2003-12-01', '宁波', 'qiushi@campus.test');

INSERT INTO course(cid, cname, tid, credit) VALUES
('C01', '数据库原理', 'T02', 4),
('C02', '离散数学', 'T01', 3),
('C03', '操作系统', 'T03', 4),
('C04', '计算机网络', 'T02', 3),
('C05', '编译原理', 'T01', 4),
('C06', '线性代数', 'T03', 3),
('C07', '软件工程', 'T04', 2),
('C08', '人工智能', 'T04', 3),
('C09', '毕业实习', 'T03', 1);

INSERT INTO score(sid, cid, score, exam_on) VALUES
('S01', 'C01', 88, '2025-01-08'),
('S01', 'C02', 76, '2025-01-10'),
('S01', 'C03', 91, '2025-01-12'),
('S01', 'C04', 85, '2025-01-09'),
('S01', 'C05', 72, '2025-01-11'),
('S01', 'C06', 90, '2025-01-13'),
('S01', 'C07', 68, '2025-01-08'),
('S01', 'C08', 80, '2025-02-15'),
('S02', 'C01', 70, '2025-01-08'),
('S02', 'C02', 82, '2025-01-10'),
('S02', 'C03', 64, '2025-01-12'),
('S02', 'C04', 55, '2025-01-09'),
('S03', 'C01', 90, '2025-01-08'),
('S03', 'C02', 90, '2025-01-10'),
('S03', 'C03', 85, '2025-01-12'),
('S03', 'C05', 78, '2025-01-11'),
('S04', 'C01', 45, '2025-01-08'),
('S04', 'C02', 52, '2025-01-10'),
('S04', 'C03', 40, '2025-01-12'),
('S04', 'C04', 0, '2025-01-09'),
('S05', 'C01', 79, '2025-01-08'),
('S05', 'C02', 88, '2025-01-10'),
('S05', 'C03', 70, '2025-01-09'),
('S06', 'C01', 0, '2025-01-08'),
('S06', 'C03', 58, '2025-01-12'),
('S06', 'C06', 41, '2025-01-13'),
('S07', 'C02', 94, '2025-01-10'),
('S07', 'C03', 90, '2025-01-12'),
('S07', 'C07', 88, '2025-01-11'),
('S08', 'C01', 86, '2025-01-08'),
('S08', 'C02', 61, '2025-01-10'),
('S08', 'C03', 73, '2025-01-12'),
('S08', 'C08', 69, '2025-02-15'),
('S09', 'C01', 92, '2025-01-08'),
('S09', 'C03', 81, '2025-01-12'),
('S09', 'C04', 77, '2025-01-09'),
('S10', 'C02', 55, '2025-01-10'),
('S10', 'C03', 47, '2025-01-12'),
('S11', 'C01', 77, '2025-01-08'),
('S11', 'C02', 80, '2025-01-10'),
('S11', 'C03', 79, '2025-01-12'),
('S11', 'C05', 66, '2025-01-11'),
('S12', 'C01', 60, '2025-01-08'),
('S13', 'C02', 84, '2025-01-10'),
('S13', 'C07', 91, '2025-01-11'),
('S14', 'C01', 58, '2025-01-08'),
('S14', 'C06', 73, '2025-01-13'),
('S15', 'C03', 62, '2025-01-12'),
('S15', 'C08', 0, '2025-02-15');

INSERT INTO dept(did, dname) VALUES
('D01', '研发部'),
('D02', '教学部'),
('D03', '市场部');

INSERT INTO emp(eid, ename, did, manager_id, salary, hired_on) VALUES
('E01', '陈省', 'D01', NULL, 18000, '2020-03-01'),
('E02', '吴舟', 'D01', 'E01', 12000, '2022-05-12'),
('E03', '唐宁', 'D01', 'E01', 19000, '2021-08-20'),
('E04', '沈泊', 'D02', 'E01', 15000, '2021-11-04'),
('E05', '齐末', 'D02', 'E04', 9000, '2024-04-01'),
('E06', '米白', 'D03', 'E01', 11000, '2023-02-18'),
('E07', '蓝秋', 'D03', 'E06', 8000, '2024-09-01'),
('E08', '贺川', 'D03', 'E06', 21000, '2022-12-10');

-- 本题查询
SELECT ROUND(100.0 * SUM(CASE WHEN course_cnt >= 2 THEN 1 ELSE 0 END) / COUNT(*), 2) AS multi_course_pct
FROM (SELECT sid, COUNT(DISTINCT cid) AS course_cnt FROM score GROUP BY sid) x;

运行示例

multi_course_pct
----------------
93.33