PostgreSQL Specify the same field to find the last set of records

The title is quite tangled, or an example.

Is that I have a bunch of records,

created_date| Key | Content |
1 | Test| Xx?xxxxx|
2 | Test| Xxxxx?xx|
3 | Test|? xxxxxxx|
2 | Hah | xxxxxxx?|
3 | Hah | Xxxx?xxx|

I want to remove the key, which is test and haha, respectively.

created_date| Key | Content |
3 | Test|? xxxxxxx|
3 | Hah | Xxxx?xxx|

I can't use group because I want content.

Finally the problem is described clearly, SQL is tired

Can be solved this way (I used to brush under the oralce exist, now quickly forgotten):

SELECT *
From comment C
WHERE not EXISTS (
SELECT *
From Comment C2
WHERE C.key = C2.key
and C2.created_date > C.created_date
);