聚合函数与分组

-- 统计总数
SELECT COUNT(*) AS 学生总数 FROM students;

-- 平均值、最大、最小
SELECT
    AVG(score) AS 平均分,
    MAX(score) AS 最高分,
    MIN(score) AS 最低分
FROM students;

-- 按年龄分组统计人数
SELECT age, COUNT(*) AS 人数
FROM students
GROUP BY age;

-- 分组 + 过滤（HAVING）
SELECT age, AVG(score) AS 平均分
FROM students
GROUP BY age
HAVING AVG(score) > 80;

-- 不重复统计
SELECT COUNT(DISTINCT age) AS 不同年龄数 FROM students;

-- 分组统计 + 排序
SELECT age, COUNT(*) AS 人数
FROM students
GROUP BY age
ORDER BY 人数 DESC;