generated.md

# 生成列

Joe 需要为 Points 表

```sql
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？

<br>

（提示）MySQL生成列的语法结构为：

```sql
column_name data_type [GENERATED ALWAYS] AS (expression)
```

<hr/>

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

## 答案

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

## 选项

### A

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

### B

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

### C

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