# nullIf
<a name="nullIf-function"></a>

`nullIf` compares two expressions. If they are equal, the function returns null. If they are not equal, the function returns the first expression.

## Syntax
<a name="nullIf-function-syntax"></a>

```
nullIf(expression1, expression2)
```

## Arguments
<a name="nullIf-function-arguments"></a>

`nullIf` takes two expressions as arguments. 

 *expression*   
The expression can be numeric, datetime, or string. It can be a field name, a literal value, or another function. 

## Return type
<a name="nullIf-function-return-type"></a>

String

## Example
<a name="nullIf-function-example"></a>

The following example returns nulls if the reason for a shipment delay is unknown.

```
nullIf(delayReason, 'unknown')
```

The following are the given field values.

```
delayReason
============
unknown         
back ordered 
weather delay
```

For these field values, the following values are returned.

```
(null)
back ordered 
weather delay
```