### STDDEV_POP Population Standard Deviation Function
```sql
stddev_pop([distinct] expr)
```
#### Function Description
The `STDDEV_POP` function is used to calculate the population standard deviation of a set of numerical data, which measures the dispersion of values in the dataset. A smaller standard deviation indicates that the data is more concentrated, while a larger standard deviation indicates that the data is more dispersed.

#### Parameter Description
- `expr`: Must be of a numerical type, including tinyint, smallint, int, bigint, float, double, and decimal. The function will calculate the value of this expression for each row.
- `distinct` (optional): When using the `distinct` keyword, the function will calculate the standard deviation of the unique set of values. If this keyword is omitted, the function will calculate the standard deviation of all values, including duplicates.

#### Return Type
The return type is double.

#### Notes
- `null` values in the function will not be included in the standard deviation calculation.
- When the dataset is empty, the function returns `null`.

#### Usage Example

**Example 1:** Calculate the population standard deviation of a set of values
```sql
SELECT stddev_pop(col) FROM VALUES (1), (2), (3), (3), (null) AS tab(col);
-- Result: 0.82915619758885
```
In this example, we calculate the standard deviation of a numerical set that includes duplicate values and `null` values.

**Example 2:** Calculate the standard deviation of a numerical set after removing duplicates
```sql
SELECT stddev_pop(DISTINCT col) FROM VALUES (1), (2), (3), (3), (null) AS tab(col);
-- Result: 0.816496580927726
```
In this example, we calculate the standard deviation of a set of values after removing duplicates by using the `distinct` keyword.

**Example 3:** Calculating the standard deviation of a numeric column in an actual data table

Suppose we have a data table named `orders`, which contains a numeric column named `amount` representing the order amount. We can use the `STDDEV_POP` function to calculate the overall standard deviation of the order amounts.
```sql
SELECT stddev_pop(amount) FROM orders;
-- Result: XXX.XXX
```
In this example, we calculate the standard deviation of all order amounts in the `orders` table to understand the dispersion of order amounts.

**Example 4:** Calculate the standard deviation of deduplicated order amounts

If we want to calculate the standard deviation of deduplicated order amounts, we can use the `STDDEV_POP` function like this:
```sql
SELECT stddev_pop(DISTINCT amount) FROM orders;
-- Result: XXX.XXX
```
In this example, we calculate the standard deviation of the set of values after removing duplicate order amounts by using the `distinct` keyword. This can help us better understand the distribution of order amounts.