The schema of the tables, how to organize a rating with pros/cons?

Posts_users_rel intermediate table with the structure: id int, post_id int, user_id int
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In the vote of the visitor is added to the table entry that contains the id of the post for which visitor votes and the id of the visitor.
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Verification will be very simple.
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This table and the assessment of the visitor insert minusanul (-1) or put (+1), then the total rating of the post is simply calculated by the sum of the scores in this field.

I have done so
the table articles contains the field avg_rate — in it we write the mean value of the votes
ranking table — id articles qty — the number of votes divided by the sum of the votes
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when voting for the article, we:
1. add 1 to the qty field in the rating table
2. add the rating to the field sum sum
3. pisitive average and write it in the table of articles
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Why write? And then to be a little easier to work with your articles.