SQL 第52题：统计成绩等级人数

题目

将成绩分为不及格低于60、及格60到80不含80、优秀80及以上，统计各等级人数。

知识点：CASE、GROUP BY

运行环境：本站提供 SQLite 环境，下面脚本可直接运行。若使用 MySQL，见文中对照；有差异的题目会同时给出 MySQL 写法。

思路

先按分数分档：低于 60 不及格，60 到 80 不含 80 算及格，80 及以上优秀。用 CASE WHEN 自上而下，命中就停。

再按档次 GROUP BY 计数。SQLite 要把同一段 CASE 在 SELECT 和 GROUP BY 里各写一次。

MySQL 与 SQLite 对照

SQLite 解法

-- 第52题：统计成绩等级人数
-- 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 CASE
  WHEN score < 60 THEN '不及格'
  WHEN score < 80 THEN '及格'
  ELSE '优秀'
END AS grade, COUNT(*) AS cnt
FROM score
GROUP BY CASE
  WHEN score < 60 THEN '不及格'
  WHEN score < 80 THEN '及格'
  ELSE '优秀'
END
ORDER BY cnt DESC;

MySQL 解法

查询部分与 SQLite 的差异如下（建表请改用 VARCHAR / DECIMAL / DATE）。

SELECT IF(score < 60, '不及格', IF(score < 80, '及格', '优秀')) AS grade, COUNT(*) AS cnt
FROM score
GROUP BY grade
ORDER BY cnt DESC;

运行示例

grade | cnt
------+----
优秀    | 19 
及格    | 18 
不及格   | 12