如何在mysql中基于2列实现唯一键?

I have a table which is updated hourly. It stores of the data of Air Quality Index of different cities. The structure is like this:

id | city_code |       date_time       | pollution_level
--   ---------         ---------        ---------------
1       100       2017-02-06 13:00:00         349
2       150       2017-02-06 13:00:00         245
3       100       2017-02-06 14:00:00         325
4       150       2017-02-06 14:00:00         216

Now the problem is, I want to add a unique key in such a way that it should store data of particular city only once in same hour. Pollution level of city 100 at 2017-02-06 13:00:00 should not be there more than once in the table. Is there any way I can do it within database or any other good solution with PHP.