### VAR_POP Function
```sql
VAR_POP([DISTINCT] expr)
```
#### Function Description
The VAR_POP function is used to calculate the population variance of a set of numerical data. Variance is a statistical measure of the dispersion of data distribution, used to reflect the volatility of data. By calculating the population variance, we can understand the range of data fluctuations and trends.

#### Parameter Description
* expr (required): The numerical type field for which the variance needs to be calculated. It can be of type tinyint, smallint, int, bigint, float, double, or decimal.
* DISTINCT (optional): When set to DISTINCT, the function will calculate the variance of the deduplicated set. If DISTINCT is not set, the default is to calculate the population variance including duplicate values.

#### Return Result
* Returns a double type numerical result.
* When the input data contains null values, null does not participate in the variance calculation.

#### Usage Example

**Example 1: Basic Usage**
```sql
SELECT VAR_POP(col) FROM VALUES (1), (2), (3), (3), (null) AS tab(col);
```
Results:
```
0.6875
```
In the above example, we calculated the population variance including duplicate values and null values.

**Example 2: Using the DISTINCT keyword**
```sql
SELECT VAR_POP(DISTINCT col) FROM VALUES (1), (2), (3), (3), (null) AS tab(col);
```
Results:
```
0.6666666666666666
```
In this example, we set the DISTINCT keyword to calculate the variance of the deduplicated set.

**Example 3: Calculate the overall variance of student grades**
```sql
CREATE TABLE students (id INT, score DECIMAL(5, 2));
INSERT INTO students (id, score) VALUES (1, 85.5), (2, 92.3), (3, 90.2), (4, 88.5), (5, 95.5);
SELECT VAR_POP(score) FROM students;
```
Results:
```
11.32666666666667
```
In this example, we create a student grade table and insert some data, then use the VAR_POP function to calculate the population variance of the grades.

**Example 4: Calculate the population variance excluding a certain grade**
```sql
SELECT VAR_POP(DISTINCT score) FROM students WHERE score < 90;
```
Results:
```
8.96
```
In this example, we use the DISTINCT keyword to exclude duplicate scores and calculate the overall variance of scores less than 90.

Through the above example, you can better understand the usage and function of the VAR_POP function. In practical applications, you can choose whether to use the DISTINCT keyword and how to handle null values according to your needs.