The *NULLIF* function in SQL is used to return *NULL* when two expressions are equal; otherwise, it returns the value of the first expression. The syntax for *NULLIF* is as follows:
NULLIF(expression1, expression2)

- *expression1*: The first expression to evaluate.
- *expression2*: The second expression to compare against *expression1*.
If *expression1* and *expression2* evaluate to the same value, *NULLIF* returns *NULL*. If they do not, it returns the value of *expression1*.
###### Example

Here’s an example to illustrate how to use *NULLIF*. Let's say you have a table named *orders* with a column *discount*. You want to return the discount, but if the discount is 0, you want it to return *NULL*.
SELECT
order_id,
NULLIF(discount, 0) AS discount_value
FROM
orders;

In this example:
- If *discount* is 0, the *NULLIF* function returns *NULL*.
- If *discount* has any other value, it returns that value.
###### Real Example with Condition

You can also use *NULLIF* in a more complex query or calculation. For instance, if you want to compute the average discount but replace 0 with *NULL*:
SELECT
AVG(NULLIF(discount, 0)) AS average_discount
FROM
orders;

In this case, any 0 discounts will be treated as *NULL* in the calculation of the average, potentially giving you a more accurate result.
By using *NULLIF*, you can effectively handle cases where you want to avoid using specific values in your calculations or results.