## IFNULL 

The IFNULL function is a logical function used to handle NULL values that may appear in SQL queries. When the value of the first parameter is NULL, the IFNULL function returns the value of the second parameter; if the value of the first parameter is not NULL, it returns the value of the first parameter.

### Syntax
```sql
IFNULL(expression_1, expression_2)
```
- `expression_1`: The expression to be checked, usually a field name.
- `expression_2`: The value to return when `expression_1` is NULL.

### Instructions

- The data types of `expression_1` and `expression_2` must be the same or compatible.
- If the data types of `expression_1` and `expression_2` are not compatible, the IFNULL function will return NULL.
- The return value of the IFNULL function is the data type of `expression_1` or `expression_2`.

### Example

Assume there is a table named `student` with the following columns:

- `id`: The unique identifier of the student.
- `name`: The name of the student.
- `score`: The score of the student.

The data in the `student` table is as follows:

| id | name  | score |
|----|-------|-------|
| 1  | Alice | 90    |
| 2  | Bob   | NULL  |
| 3  | Cathy | 80    |
| 4  | David | NULL  |

Now, we want to query the name and score of each student, replacing records with NULL scores with 0. We can use the following SQL statement:
```sql
SELECT name, IFNULL(score, 0) AS score FROM student;
```
The query results are as follows:

| name  | score |
|-------|-------|
| Alice | 90    |
| Bob   | 0     |
| Cathy | 80    |
| David | 0     |

### More Examples

1. Query the student's name and age. If the age is NULL, display it as "Unknown":
```sql
SELECT name, IFNULL(age, 'Unknown') AS age FROM student;
```
2. Query the employee's name and salary. If the salary is NULL, display it as "Not available":
```sql
SELECT name, IFNULL(salary, 'Not available') AS salary FROM employee;
```

3. Query the product name and inventory quantity. If the inventory quantity is NULL, display as "Out of stock":
```sql
SELECT product_name, IFNULL(stock_quantity, 'Out of stock') AS stock FROM products;
```
