generated.md

# 生成列

Joe 需要为 Points 表

```mysql
create table points(
    id int primary key auto_increment,
    x float,
    y float
)
```

增加一个生成列，保存每个点的模（modulus），即 `√(x^2 + y^2)` 。下面哪个操作是正确的？

## 答案

```mysql
alter table points add modulus double generated always as (sqrt(x*x + y*y));
```

## 选项

### A

```mysql
create generated modulus on table points as (sqrt(x*x + y*y));
```

### B

```mysql
alter table points add modulus generated always as (sqrt(x*x + y*y));
```

### C

```mysql
alter table points add modulus float generated sqrt(x*x + y*y);
```