编写一个函数，将给定的字符串中的所有单词按字母顺序排序，并返回字符串。


字符串："hello world, how are you?"
```python
def sort_words(string):
    words = string.split()
    sorted_words = sorted(words)
    return " ".join(sorted_words)

string = "hello world, how are you?"
sorted_string = sort_words(string)
print(sorted_string)
```

输出:
```
are hello how world, you?
```