### LN 

####  Description
The LN function is used to calculate the natural logarithm (logarithm to the base e) of a given numerical expression. Natural logarithms have wide applications in fields such as mathematics, physics, and engineering, for example, when dealing with problems of exponential growth and decay.

#### Syntax
```
LN(expr)
```
#### Parameters
- `expr`: The numerical expression for which the natural logarithm needs to be calculated, of type double.

#### Return Value
Returns the calculated result, of type double.

#### Example Usage
1. Calculate the natural logarithm of a constant value:
   ```sql
   SELECT LN(3.1415926);
   ```
Result:
   ```
   1.144729868791239
   ```
2. Calculate the natural logarithm of a negative number (result is NULL):
   ```sql
   SELECT LN(-1);
   ```
Result:
   ```
   null
   ```
3. Calculate the natural logarithm of a larger number:
   ```sql
   SELECT LN(1000000);
   ```
Result:
   ```
    13.815510557964274
   ```
4. Calculate the natural logarithm of e (the result should be close to 1):
   ```sql
   SELECT LN(EXP(1));
   ```
Result:
   ```
   1.0
   ```
#### Notes
- When the `expr` parameter is negative, the LN function will return NULL because negative numbers do not have a natural logarithm in the real number domain.
- When the `expr` parameter is 0, the LN function will also return NULL because 0 does not have a natural logarithm.
- When using the LN function, please ensure that the passed expression is a valid double type value, otherwise it may result in incorrect calculations or return NULL.