Update max.md

docs/en/sql-reference/aggregate-functions/reference/max.md

@@ -4,4 +4,22 @@ toc_priority: 3
 
 # max {#agg_function-max}
 
-Calculates the maximum.
+
+Aggregate function that calculates the maximum across a group of values.
+
+Example:
+
+```
+SELECT max(salary) FROM employees;
+```
+
+```
+SELECT department, max(salary) FROM employees GROUP BY department;
+```
+
+If you need non-aggregate function to choose a maximum of two values, see `greatest`:
+
+```
+SELECT greatest(a, b) FROM table;
+```
+