MySQL - 基础增删查改

以下是 MySQL 数据库基础的增删查改（CRUD）操作详解：

一、数据库操作

1. 创建数据库

CREATE DATABASE db_name;
CREATE DATABASE IF NOT EXISTS db_name;  -- 不存在时才创建
CREATE DATABASE db_name CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

2. 删除数据库

DROP DATABASE db_name;
DROP DATABASE IF EXISTS db_name;  -- 存在时才删除

3. 选择/切换数据库

USE db_name;

二、数据表操作

1. 创建表

CREATE TABLE users (
    id INT PRIMARY KEY AUTO_INCREMENT,        -- 主键，自增
    username VARCHAR(50) NOT NULL UNIQUE,     -- 非空，唯一
    password VARCHAR(255) NOT NULL,
    email VARCHAR(100),
    age INT DEFAULT 18,                       -- 默认值
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);

2. 删除表

DROP TABLE table_name;
DROP TABLE IF EXISTS table_name;

3. 修改表结构

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

-- 修改列
ALTER TABLE users MODIFY COLUMN email VARCHAR(150);

-- 重命名列
ALTER TABLE users CHANGE COLUMN phone mobile VARCHAR(20);

-- 删除列
ALTER TABLE users DROP COLUMN mobile;

-- 添加索引
CREATE INDEX idx_username ON users(username);

-- 删除索引
DROP INDEX idx_username ON users;

三、数据操作（CRUD）

1. 插入数据（Create）

-- 插入单条数据（指定列）
INSERT INTO users (username, password, email, age) 
VALUES ('john_doe', '123456', 'john@example.com', 25);

-- 插入单条数据（所有列）
INSERT INTO users VALUES (NULL, 'jane_doe', '654321', 'jane@example.com', 30, NOW(), NOW());

-- 插入多条数据
INSERT INTO users (username, password, email) 
VALUES 
    ('alice', 'pass123', 'alice@example.com'),
    ('bob', 'pass456', 'bob@example.com'),
    ('charlie', 'pass789', 'charlie@example.com');

2. 查询数据（Read）

-- 查询所有列
SELECT * FROM users;

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

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

-- 逻辑运算符
SELECT * FROM users WHERE age > 20 AND email LIKE '%@example.com';
SELECT * FROM users WHERE age < 18 OR age > 60;

-- 排序
SELECT * FROM users ORDER BY age DESC;        -- 降序
SELECT * FROM users ORDER BY created_at ASC;  -- 升序

-- 限制返回数量
SELECT * FROM users LIMIT 10;                 -- 前10条
SELECT * FROM users LIMIT 5, 10;              -- 跳过5条，取10条

-- 去重
SELECT DISTINCT age FROM users;

-- 分组统计
SELECT age, COUNT(*) as count FROM users GROUP BY age;
SELECT age, COUNT(*) FROM users GROUP BY age HAVING COUNT(*) > 1;

-- 聚合函数
SELECT COUNT(*) as total_users FROM users;
SELECT AVG(age) as avg_age FROM users;
SELECT MAX(age) as max_age FROM users;
SELECT MIN(age) as min_age FROM users;
SELECT SUM(age) as total_age FROM users;

3. 更新数据（Update）

-- 更新单个字段
UPDATE users SET email = 'new_email@example.com' WHERE id = 1;

-- 更新多个字段
UPDATE users SET 
    email = 'updated@example.com',
    age = 26,
    updated_at = NOW()
WHERE username = 'john_doe';

-- 基于现有值更新
UPDATE users SET age = age + 1 WHERE age < 30;

-- 批量更新
UPDATE users SET status = 'inactive' WHERE last_login < '2024-01-01';

4. 删除数据（Delete）

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

-- 删除多条记录
DELETE FROM users WHERE age > 100;

-- 清空表（删除所有记录）
DELETE FROM users;
TRUNCATE TABLE users;  -- 更快，不能回滚

四、高级查询技巧

1. 连接查询

-- 内连接
SELECT u.username, o.order_id, o.amount
FROM users u
INNER JOIN orders o ON u.id = o.user_id;

-- 左连接
SELECT u.username, o.order_id
FROM users u
LEFT JOIN orders o ON u.id = o.user_id;

-- 右连接
SELECT u.username, o.order_id
FROM users u
RIGHT JOIN orders o ON u.id = o.user_id;

2. 子查询

-- WHERE子句中的子查询
SELECT * FROM users 
WHERE age > (SELECT AVG(age) FROM users);

-- FROM子句中的子查询
SELECT avg_age_table.age_group, COUNT(*)
FROM (SELECT 
    CASE 
        WHEN age < 20 THEN '少年'
        WHEN age BETWEEN 20 AND 40 THEN '青年'
        ELSE '中老年'
    END as age_group
    FROM users) as avg_age_table
GROUP BY age_group;

3. 联合查询

SELECT username, email FROM active_users
UNION
SELECT username, email FROM inactive_users;

五、事务处理

-- 开始事务
START TRANSACTION;

-- 执行操作
UPDATE accounts SET balance = balance - 100 WHERE user_id = 1;
UPDATE accounts SET balance = balance + 100 WHERE user_id = 2;

-- 提交或回滚
COMMIT;  -- 确认更改
-- 或
ROLLBACK;  -- 撤销更改

六、实用技巧

1. 备份数据

-- 创建表备份
CREATE TABLE users_backup AS SELECT * FROM users;

-- 插入备份数据
INSERT INTO users_backup SELECT * FROM users WHERE created_at > '2024-01-01';

2. 批量操作

-- 批量插入（提高性能）
INSERT INTO users (username, email) VALUES 
('user1', 'user1@example.com'),
('user2', 'user2@example.com'),
('user3', 'user3@example.com');

3. 使用CASE表达式

SELECT 
    username,
    age,
    CASE 
        WHEN age < 18 THEN '未成年'
        WHEN age BETWEEN 18 AND 60 THEN '成年'
        ELSE '老年'
    END as age_group
FROM users;

注意事项

1. WHERE条件：UPDATE和DELETE操作一定要加WHERE条件，除非确实要操作所有记录
2. 事务处理：重要操作使用事务，确保数据一致性
3. 索引使用：频繁查询的字段建议添加索引
4. SQL注入：应用程序中要使用参数化查询防止SQL注入
5. 性能优化：大数据量时注意查询性能，避免SELECT *