unique.md

# 唯一约束

Goods 表结构如下：

```mysql
create table goods(
    id int primary key auto_increment,
    category_id int,
    name varchar(256),
    price decimal(12, 4),
    stock int,
    upper_time timestamp
)
```

Joe 需要确保同一个类型下没有重名的商品，他应该怎么做？

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* `show databases;` 列出所有数据库
* `show tables;` 列出所有表

## 答案

```mysql
alter table goods add unique index (category_id, name);
```

## 选项

### A

```mysql
alter table goods add index (category_id, name);
```

### B

```mysql
alter table goods add unique index (category_id + name);
```

### C

```mysql
alter table goods add unique index (concat(category_id, name));
```