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+# FastAPI – Query Parameters
+
+This document demonstrates how to use **query parameters** in FastAPI using multiple typing styles, including:
+
+* Native Python union types (`str | None`)
+* `Optional` from `typing`
+* `Annotated` with `Query` validation
+
+---
+
+## Example Application
+
+Create or update `main.py` with the following content:
+
+```python
+from fastapi import FastAPI, Query
+from typing import Optional, Annotated
+
+app = FastAPI()
+
+users = [
+    {"name": "abbas", "age": 20},
+    {"name": "abbas", "age": 32},
+    {"name": "abbas", "age": 54},
+    {"name": "abbas", "age": 15},
+    {"name": "mmd", "age": 37},
+    {"name": "asghar", "age": 19},
+]
+
+
+@app.get("/")
+def home_page():
+    return {"msg": "API is working"}
+
+
+@app.get("/user")
+def get_users(target_name: str | None = None):
+    if target_name:
+        return [item for item in users if item["name"] == target_name]
+    return users
+
+
+@app.get("/user2")
+def get_users_optional(target_name: Optional[str] = None):
+    if target_name:
+        return [item for item in users if item["name"] == target_name]
+    return users
+
+
+@app.get("/user3")
+def get_users_annotated(
+    target_name: Annotated[str | None, Query(max_length=50)] = None
+):
+    if target_name:
+        return [item for item in users if item["name"] == target_name]
+    return users
+```
+
+---
+
+## Query Parameter Overview
+
+Query parameters are key-value pairs appended to the URL after `?`.
+
+**Example:**
+
+```
+/user?target_name=abbas
+```
+
+---
+
+## Defined Endpoints
+
+### 1. Basic Query Parameter (Python Union Type)
+
+```python
+@app.get("/user")
+def get_users(target_name: str | None = None):
+```
+
+* Uses Python 3.10+ union syntax
+* `target_name` is optional
+* If not provided, all users are returned
+
+**Example Request:**
+
+```
+GET /user?target_name=abbas
+```
+
+---
+
+### 2. Optional Query Parameter (`typing.Optional`)
+
+```python
+@app.get("/user2")
+def get_users_optional(target_name: Optional[str] = None):
+```
+
+* Uses `Optional[str]` for compatibility with older Python versions
+* Behavior is identical to the first endpoint
+
+**Example Request:**
+
+```
+GET /user2?target_name=abbas
+```
+
+---
+
+### 3. Validated Query Parameter (`Annotated + Query`)
+
+```python
+@app.get("/user3")
+def get_users_annotated(
+    target_name: Annotated[str | None, Query(max_length=50)] = None
+):
+```
+
+* Uses `Annotated` for metadata binding
+* Adds validation rules:
+
+  * `max_length=50`
+* Automatically returns validation errors if constraints are violated
+
+**Example Invalid Request:**
+
+```
+GET /user3?target_name=verylongnamethatexceedslimit...
+```
+
+**Response:**
+
+```json
+{
+  "detail": [
+    {
+      "type": "string_too_long",
+      "loc": ["query", "target_name"],
+      "msg": "String should have at most 50 characters",
+      "input": "..."
+    }
+  ]
+}
+```
+
+---
+
+## Example Responses
+
+### Filtered Response
+
+```json
+[
+  {"name": "abbas", "age": 20},
+  {"name": "abbas", "age": 32},
+  {"name": "abbas", "age": 54},
+  {"name": "abbas", "age": 15}
+]
+```
+
+---
+
+### Default Response (No Query Parameter)
+
+```json
+[
+  {"name": "abbas", "age": 20},
+  {"name": "abbas", "age": 32},
+  {"name": "abbas", "age": 54},
+  {"name": "abbas", "age": 15},
+  {"name": "mmd", "age": 37},
+  {"name": "asghar", "age": 19}
+]
+```
+
+---
+
+## Running the Application
+
+Start the service using `uvicorn`:
+
+```bash
+uvicorn main:app --reload
+```
+
+---
+
+## Best Practices
+
+* Use query parameters for filtering and searching
+* Always provide defaults for optional parameters
+* Use `Query()` for validation and constraints
+* Return full datasets when no filters are applied
+* Avoid large in-memory datasets in production
+* Use pagination for large result sets
+* Combine query parameters with database queries in real systems
+