Update 03-udf.md

Author: soyeric128

docs/en/guides/54-query/03-udf.md

@@ -57,8 +57,11 @@ Embedded UDFs allow you to embed code written in the following programming langu
 - [JavaScript](#javascript)
 - [WebAssembly](#webassembly)
 
+With Embedded UDFs, you can create both scalar functions and aggregate functions. Scalar functions operate on a single row of input and return a single value, while aggregate functions process multiple rows of input and return a single aggregated result, such as a sum or average.
+
 :::note
-If your program content is large, you can compress it and then pass it to a stage. See the [Usage Examples](#usage-examples-2) for WebAssembly.
+- Creating aggregate UDFs with WebAssembly is not yet supported.
+- If your program content is large, you can compress it and then pass it to a stage. See the [Usage Examples](#usage-examples-2) for WebAssembly.
 :::
 
 ### Python (requires Databend Enterprise)
@@ -237,6 +240,35 @@ WHERE
 ORDER BY 1;
 ```
 
+This example defines an aggregate UDF that calculates the weighted average of a set of values by aggregating them based on their corresponding weights:
+
+```sql
+CREATE FUNCTION weighted_avg (INT, INT) STATE {sum INT, weight INT} RETURNS FLOAT
+LANGUAGE javascript AS $$
+export function create_state() {
+    return {sum: 0, weight: 0};
+}
+export function accumulate(state, value, weight) {
+    state.sum += value * weight;
+    state.weight += weight;
+    return state;
+}
+export function retract(state, value, weight) {
+    state.sum -= value * weight;
+    state.weight -= weight;
+    return state;
+}
+export function merge(state1, state2) {
+    state1.sum += state2.sum;
+    state1.weight += state2.weight;
+    return state1;
+}
+export function finish(state) {
+    return state.sum / state.weight;
+}
+$$;
+```
+
 ### WebAssembly
 
 A WebAssembly UDF allows users to define custom logic or operations using languages that compile to WebAssembly. These UDFs can then be invoked directly within SQL queries to perform specific computations or data transformations.