How do you use the IFNULL function in SQL?

The IFNULL function in SQL is used to return an alternative value if the first expression is NULL.

The IFNULL function is a MySQL function which allows you to provide two arguments, and it will return the first argument if it is not NULL, otherwise it will return the second argument. The syntax for the IFNULL function is as follows: IFNULL(expression1, expression2). Here, expression1 is the value to test whether it is NULL or not, and expression2 is the value to return if expression1 is NULL.

For example, consider a table named 'Students' with a column 'Grade'. Some students might not have a grade yet, so their grade is NULL. If you want to select all students' grades, but for those who don't have a grade yet, you want to display 'Not Graded', you can use the IFNULL function like this: SELECT IFNULL(Grade, 'Not Graded') AS Grade FROM Students. This will return the Grade if it is not NULL, otherwise it will return 'Not Graded'.

The IFNULL function is very useful in handling NULL values in SQL. It helps to prevent errors that can occur when performing operations on NULL values. For instance, if you try to add a number to a NULL value, the result will be NULL. But with the IFNULL function, you can provide a default value to use in case of NULL, thus avoiding such issues.

Remember, the IFNULL function is specific to MySQL. Other databases have similar functions to handle NULL values, like ISNULL in SQL Server, NVL in Oracle, and COALESCE in PostgreSQL, which can be used in a similar way. It's important to know the specific functions and syntax for the database system you are using.