查询（SELECT）

条件查询：SELECT * FROM users WHERE age > 18;

模糊查询：SELECT * FROM users WHERE name LIKE '%Tom%';

IN 查询：SELECT * FROM users WHERE id IN (1,2,3);

BETWEEN 查询：SELECT * FROM users WHERE age BETWEEN 18 AND 30;

去重：SELECT DISTINCT name FROM users;

插入（INSERT）

插入一条数据：INSERT INTO users (name, age) VALUES ('Tom', 20);

批量插入：INSERT INTO users (name, age) VALUES ('Tom',20),('Jerry',22);

更新（UPDATE）

更新单条：UPDATE users SET age = 25 WHERE id = 1;

批量更新：UPDATE users SET age = age + 1;

删除（DELETE）

删除单条：DELETE FROM users WHERE id = 1;

删除全部（慎用）：DELETE FROM users;

聚合函数

统计数量 COUNT：SELECT COUNT(*) FROM users;

最大值 MAX：SELECT MAX(age) FROM users;

最小值 MIN：SELECT MIN(age) FROM users;

平均值 AVG：SELECT AVG(age) FROM users;

求和 SUM：SELECT SUM(age) FROM users;

分组与筛选

分组 GROUP BY：SELECT age, COUNT(*) FROM users GROUP BY age;

筛选 HAVING：SELECT age, COUNT(*) FROM users GROUP BY age HAVING COUNT(*) > 1;

连接查询（JOIN）

内连接 INNER JOIN：SELECT * FROM orders o INNER JOIN users u ON o.user_id = u.id;

左连接 LEFT JOIN：SELECT * FROM users u LEFT JOIN orders o ON u.id = o.user_id;

表操作

创建表：CREATE TABLE users (id INT PRIMARY KEY AUTO_INCREMENT, name VARCHAR(50));

添加字段：ALTER TABLE users ADD age INT;

删除表：DROP TABLE users;

索引

创建索引：CREATE INDEX idx_name ON users(name);

删除索引：DROP INDEX idx_name ON users;

时间函数

当前时间：SELECT NOW();

格式化时间：SELECT DATE_FORMAT(NOW(), '%Y-%m-%d %H:%i:%s');