### ASIN 

#### Function Description
The `asin` function is used to calculate the arcsine of a given number, which is the angle whose sine is equal to the given value. This function is widely used in fields such as mathematics, physics, and engineering.

#### Syntax
```
asin(expr)
```
#### Parameters
- `expr`: The value for which the arcsine needs to be calculated, of type double.

#### Return Value
Returns the calculated arcsine value, of type double.

#### Usage Instructions
- When the value of `expr` is between -1 and 1 (inclusive of -1 and 1), the `asin` function can return a valid arcsine value.
- When the value of `expr` is less than -1 or greater than 1, the `asin` function will return `NaN` (Not a Number).

#### Example
```
-- Calculate the arcsine of 0
> SELECT asin(0);
0.0

-- Calculate the arcsine of 0.5
> SELECT asin(0.5);
0.5235987755982989

-- Calculate the arcsine of -1
> SELECT asin(-1);
-1.5707963267948966

-- Calculate the arcsine of a value greater than 1, returns NaN
> SELECT asin(2);
NaN
```
Through the above examples, you can see the application of the `asin` function in different scenarios. Please note that when the input value is not within the range of -1 to 1, the function will return `NaN`. In practical applications, please ensure that the input values are within the valid range to avoid errors.