### ISNAN 
```
isnan(expr)
```
####  Description
The `ISNAN` function is used to determine whether the given expression `expr` is `NaN` (Not a Number). When the value of `expr` is `NaN`, the function returns `true`; otherwise, it returns `false`. This function is mainly used to handle numerical anomalies and ensure data accuracy.

#### Parameter Description
* `expr`: The expression to be evaluated, of type `double`.

#### Return Type
The return result is of type `boolean`, i.e., `true` or `false`.

#### Usage Example
1. Determine whether the string `'NaN'` is `NaN` after being converted to `double`:
```sql
SELECT isnan(cast('NaN' as double)); -- The result is true
```
2. Use the `ISNAN` function in a query to filter `NaN` values:
   ```sql
   SELECT * FROM orders WHERE isnan(total_amount);
   ```
In this example, if there are `NaN` values in the `total_amount` column, the `ISNAN` function will return `true`, thus helping to filter out these records.

3. Determine whether a calculation result is `NaN` and perform conditional filtering based on the result:
   ```sql
   SELECT * FROM products WHERE isnan(discount_price);
   ```