MySQL添加唯一索引的常见方法

CREATE TABLE table_name (
    column1 datatype UNIQUE,  -- 单列唯一索引
    column2 datatype,
    ...,
    UNIQUE (col_a, col_b)     -- 多列组合唯一索引
);

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) NOT NULL UNIQUE,  -- 单列唯一索引
    email VARCHAR(100) NOT NULL,
    phone VARCHAR(20),
    INDEX idx_email_phone (email, phone) UNIQUE  -- 组合唯一索引
);

ALTER TABLE products 
ADD UNIQUE uk_product_sku (sku);  -- 单列索引

ALTER TABLE orders 
ADD UNIQUE uk_order_user_time (user_id, order_time);  -- 组合索引

CREATE UNIQUE INDEX idx_unique_username ON customers(username);

-- 查找重复数据
SELECT col, COUNT(*) 
FROM table_name 
GROUP BY col 
HAVING COUNT(*) > 1;

-- 删除重复记录（保留最新）
DELETE t1 
FROM table_name t1
INNER JOIN table_name t2 
WHERE t1.id > t2.id AND t1.col = t2.col;

INSERT IGNORE INTO users (username) VALUES ('john');

CREATE TABLE logs (
    id INT,
    event_date DATE,
    UNIQUE (id, event_date)  -- 仅当event_date在最近30天时生效
) PARTITION BY RANGE COLUMNS(event_date);

ALTER TABLE articles ADD UNIQUE (title(50));  -- 仅索引前50字符

-- 创建商品表并添加唯一索引
CREATE TABLE products (
    id INT AUTO_INCREMENT PRIMARY KEY,
    sku VARCHAR(20) NOT NULL,
    name VARCHAR(100) NOT NULL,
    price DECIMAL(10,2) NOT NULL,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    CONSTRAINT uk_product_sku UNIQUE (sku)  -- 命名约束
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

-- 添加组合唯一索引（订单号+用户ID）
ALTER TABLE orders ADD UNIQUE uk_order_user (order_number, user_id);