add docs

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

@@ -642,6 +642,26 @@ nvl2(expression1, expression2, expression3)
 
 _Alias of [nvl](#nvl)._
 
+## Comparison Functions
+
+- [greatest](#greatest)
+
+### `greatest`
+
+Returns the greatest value in a list of expressions.
+Returns _null_ if all expressions are _null_.
+
+```
+greatest(expression1[, ..., expression_n])
+```
+
+#### Arguments
+
+- **expression1, expression_n**:
+  Expressions to compare and return the greatest value.
+  Can be a constant, column, or function, and any combination of arithmetic operators.
+  Pass as many expression arguments as necessary.
+
 ## String Functions
 
 - [ascii](#ascii)