add docs entry

docs/source/user-guide/sql/scalar_functions.md

@@ -569,6 +569,7 @@ trunc(numeric_expression[, decimal_places])
 
 - [coalesce](#coalesce)
 - [nullif](#nullif)
+- [nvl/ifnull](#nvl/ifnull)
 
 ### `coalesce`
 
@@ -603,6 +604,21 @@ nullif(expression1, expression2)
 - **expression2**: Expression to compare to expression1.
   Can be a constant, column, or function, and any combination of arithmetic operators.
 
+### `nvl/ifnull`
+
+Returns _expression2_ if _expression1_ is NULL; otherwise it returns _expression1_.
+
+```
+nullif(expression1, expression2)
+```
+
+#### Arguments
+
+- **expression1**: return if expression1 not is NULL.
+  Can be a constant, column, or function, and any combination of arithmetic operators.
+- **expression2**: return if expression1 is NULL.
+  Can be a constant, column, or function, and any combination of arithmetic operators.
+
 ## String Functions
 
 - [ascii](#ascii)