-- 查看所有数据库
SHOW DATABASES;
-- 创建数据库
CREATE DATABASE mydb;
CREATE DATABASE IF NOT EXISTS mydb;
CREATE DATABASE mydb CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;
-- 选择数据库
USE mydb;
-- 查看当前使用的数据库
SELECT DATABASE();
-- 删除数据库
DROP DATABASE mydb;
DROP DATABASE IF EXISTS mydb;

-- 基本建表语法
CREATE TABLE 表名 (
    列名 数据类型 [约束],
    列名 数据类型 [约束],
    ...
);

-- 示例：创建用户表
CREATE TABLE users (
    id INT PRIMARY KEY AUTO_INCREMENT,
    username VARCHAR(50) NOT NULL UNIQUE,
    email VARCHAR(100) NOT NULL UNIQUE,
    password VARCHAR(255) NOT NULL,
    age INT DEFAULT 0,
    created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
    updated_at DATETIME DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);

-- 查看表结构
DESC users;
-- 或
SHOW COLUMNS FROM users;

-- 查看建表语句
SHOW CREATE TABLE users;

-- 查看所有表
SHOW TABLES;

-- 添加列
ALTER TABLE users ADD COLUMN phone VARCHAR(20);

-- 修改列类型
ALTER TABLE users MODIFY COLUMN phone VARCHAR(15);

-- 修改列名和类型
ALTER TABLE users CHANGE COLUMN phone mobile VARCHAR(20);

-- 删除列
ALTER TABLE users DROP COLUMN mobile;

-- 重命名表
ALTER TABLE users RENAME TO user_accounts;

-- 删除表
DROP TABLE users;

-- 删除表（如果存在）
DROP TABLE IF EXISTS users;

-- 插入单行（指定列）
INSERT INTO users (username, email, password, age) 
VALUES ('张三', '[email protected]', '123456', 25);

-- 插入单行（所有列）
INSERT INTO users VALUES (NULL, '李四', '[email protected]', '654321', 30, NOW(), NOW());

-- 插入多行
INSERT INTO users (username, email, password, age) VALUES 
('王五', '[email protected]', '111111', 28),
('赵六', '[email protected]', '222222', 22),
('孙七', '[email protected]', '333333', 35);

-- 插入查询结果
INSERT INTO users_backup (username, email, age)
SELECT username, email, age FROM users WHERE age > 25;

-- 查询所有列
SELECT * FROM users;

-- 查询指定列
SELECT id, username, email FROM users;

-- 条件查询（WHERE）
SELECT * FROM users WHERE age > 25;
SELECT * FROM users WHERE username = '张三';
SELECT * FROM users WHERE age BETWEEN 20 AND 30;
SELECT * FROM users WHERE email LIKE '%@example.com';

-- 更新单行
UPDATE users SET age = 26 WHERE username = '张三';

-- 更新多列
UPDATE users SET age = 30, email = '[email protected]' WHERE id = 1;

-- 条件更新
UPDATE users SET age = age + 1 WHERE age < 30;

-- 更新所有行（谨慎使用）
UPDATE users SET updated_at = NOW();

-- 删除指定行
DELETE FROM users WHERE id = 1;

-- 删除满足条件的行
DELETE FROM users WHERE age > 60;

-- 删除所有行（谨慎使用）
DELETE FROM users;

-- 查看所有用户
SELECT User, Host FROM mysql.user;

-- 创建用户
CREATE USER 'username'@'localhost' IDENTIFIED BY 'password';
CREATE USER 'username'@'%' IDENTIFIED BY 'password';  -- 允许任何主机连接

-- 修改密码
ALTER USER 'username'@'localhost' IDENTIFIED BY 'new_password';
SET PASSWORD FOR 'username'@'localhost' = PASSWORD('new_password');

-- 删除用户
DROP USER 'username'@'localhost';

-- 授予权限
-- 语法：GRANT 权限 ON 数据库.表 TO '用户'@'主机'
GRANT ALL PRIVILEGES ON mydb.* TO 'username'@'localhost';
GRANT SELECT, INSERT, UPDATE ON mydb.users TO 'username'@'localhost';
GRANT SELECT ON *.* TO 'readonly'@'%';  -- 全局只读

-- 授予所有权限
GRANT ALL PRIVILEGES ON *.* TO 'admin'@'localhost' WITH GRANT OPTION;

-- 查看用户权限
SHOW GRANTS FOR 'username'@'localhost';

-- 撤销权限
REVOKE INSERT, UPDATE ON mydb.users FROM 'username'@'localhost';
REVOKE ALL PRIVILEGES ON mydb.* FROM 'username'@'localhost';

-- 刷新权限（使修改生效）
FLUSH PRIVILEGES;