ER 图

-- 示例：在线书店系统（包含标准主键、外键和联合唯一约束）
CREATE TABLE `user` (
  `id` BIGINT NOT NULL AUTO_INCREMENT COMMENT '用户ID',
  `username` VARCHAR(50) NOT NULL COMMENT '用户名',
  `email` VARCHAR(100) NOT NULL COMMENT '邮箱',
  `created_at` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
  PRIMARY KEY (`id`),
  UNIQUE KEY `uk_user_email` (`email`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='用户表';

CREATE TABLE `category` (
  `id` BIGINT NOT NULL AUTO_INCREMENT COMMENT '分类ID',
  `name` VARCHAR(80) NOT NULL COMMENT '分类名称',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='图书分类表';

CREATE TABLE `book` (
  `id` BIGINT NOT NULL AUTO_INCREMENT COMMENT '图书ID',
  `category_id` BIGINT NOT NULL COMMENT '分类ID',
  `title` VARCHAR(120) NOT NULL COMMENT '书名',
  `price` DECIMAL(10,2) NOT NULL DEFAULT 0 COMMENT '售价',
  `stock` INT NOT NULL DEFAULT 0 COMMENT '库存',
  PRIMARY KEY (`id`),
  CONSTRAINT `fk_book_category` FOREIGN KEY (`category_id`) REFERENCES `category` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='图书表';

CREATE TABLE `book_order` (
  `id` BIGINT NOT NULL AUTO_INCREMENT COMMENT '订单ID',
  `user_id` BIGINT NOT NULL COMMENT '用户ID',
  `total_amount` DECIMAL(12,2) NOT NULL DEFAULT 0 COMMENT '订单金额',
  `status` VARCHAR(20) NOT NULL DEFAULT 'pending' COMMENT '订单状态',
  `created_at` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '下单时间',
  PRIMARY KEY (`id`),
  CONSTRAINT `fk_order_user` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='订单表';

CREATE TABLE `order_item` (
  `id` BIGINT NOT NULL AUTO_INCREMENT COMMENT '明细ID',
  `order_id` BIGINT NOT NULL COMMENT '订单ID',
  `book_id` BIGINT NOT NULL COMMENT '图书ID',
  `quantity` INT NOT NULL DEFAULT 1 COMMENT '购买数量',
  `unit_price` DECIMAL(10,2) NOT NULL COMMENT '成交单价',
  PRIMARY KEY (`id`),
  UNIQUE KEY `uk_order_book` (`order_id`, `book_id`),
  CONSTRAINT `fk_item_order` FOREIGN KEY (`order_id`) REFERENCES `book_order` (`id`),
  CONSTRAINT `fk_item_book` FOREIGN KEY (`book_id`) REFERENCES `book` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='订单明细表';