### date_add Function
```sql
date_add(startDate, numDays)
```
#### Function Description
This function is used to calculate and return the date that is `numDays` days after the given `startDate`. If the calculation result exceeds the date range supported by the system, it will return `null`.

#### Parameter Description
- `startDate` (date): The start date, which needs to conform to the date format.
- `numDays` (int): The number of days to add, which can be a positive integer (to add days) or a negative integer (to subtract days).

#### Return Result
Returns a new date that is the result of adding `numDays` days to `startDate`. If the result exceeds the date range, it returns `null`.

#### Usage Example
1. Adding days:
```sql
SELECT date_add('2020-05-31', 10); -- Returns 2020-06-10
```
2. Reduce Days:
```sql
SELECT date_add('2020-06-10', -3); -- Returns 2020-06-07
```
3. Year-end Calculation:
```sql
SELECT date_add('2020-12-31', 1); -- Returns 2021-01-01
```
4. Exceeding the Date Range:
```sql
SELECT date_add('9999-12-31', 1); -- Returns null
```
#### Notes
- Ensure that the `startDate` parameter is in a valid date format, otherwise the function may return `null`.
- When `numDays` is negative, it indicates counting `numDays` days back from `startDate`.
- Please be aware of the date range supported by the system; calculations beyond this range will return `null`.