1
SELECT * FROM student ORDER BY age DESC;

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SELECT * FROM student ORDER BY class_id ASC, age DESC;

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SELECT * FROM student LIMIT 5;            -- 前 5 行
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SELECT * FROM student LIMIT 5 OFFSET 10;  -- 跳过 10 行，取 5 行

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SELECT * FROM student LIMIT 10, 5;        -- 等价于 OFFSET 10 取 5

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SELECT COUNT(*) FROM student;               -- 记录总数
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SELECT COUNT(email) FROM student;           -- email 非空的行数
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SELECT SUM(age) FROM student;               -- 年龄总和
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SELECT AVG(age) FROM student;               -- 平均年龄
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SELECT MAX(age), MIN(age) FROM student;     -- 最大、最小年龄

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SELECT class_id, COUNT(*) AS 人数 FROM student GROUP BY class_id;

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SELECT class_id, COUNT(*) AS 人数
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FROM student
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GROUP BY class_id
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HAVING COUNT(*) > 3;