R连接两个数据框，按列分组并计算平均值

Question

I've Googled around, but I can't seem to find a solution for the problem I'm having. I have two data frames, one holds movies by ID and contains ratings for them:

> summary(ratings)
    movieId        mean_rating      rating_count    
 Min.   :     1   Min.   : 1.000   Min.   :    1.0  
 1st Qu.:  6796   1st Qu.: 5.600   1st Qu.:    3.0  
 Median : 65880   Median : 6.471   Median :   18.0  
 Mean   : 58790   Mean   : 6.266   Mean   :  747.8  
 3rd Qu.: 99110   3rd Qu.: 7.130   3rd Qu.:  205.0  
 Max.   :131262   Max.   :10.000   Max.   :67310.0  
      rn           
 Length:26744      
 Class :character  
 Mode  :character  

The other one is a collection of user defined tags that have been added to these movies. It also has a column called movieId that corresponds to movieId in the first data frame.

> summary(tags)
     userId          movieId           tag           
 Min.   :    18   Min.   :     1   Length:465564     
 1st Qu.: 28780   1st Qu.:  2571   Class :character  
 Median : 70201   Median :  7373   Mode  :character  
 Mean   : 68712   Mean   : 32628                     
 3rd Qu.:107322   3rd Qu.: 62235                     
 Max.   :138472   Max.   :131258                     
   timestamp               rn           
 Min.   :1135429210   Length:465564     
 1st Qu.:1245007262   Class :character  
 Median :1302291181   Mode  :character  
 Mean   :1298711076                     
 3rd Qu.:1366217861                     
 Max.   :1427771352  

What I want to do, is get the mean movie rating for each of the tags. Basically, the equivalent of this SQL query:

SELECT t.tag, AVG(r.mean_rating) FROM movielens_tags t RIGHT JOIN movielens_ratings r ON t.movieId = r.movieId GROUP BY t.tag;

I just need 2 columns in the output:

      tag      mean_rating
sci_fi         6.23
bollywood      7.45
action         5.75

However, this SQL query will never end. That's why I want to do it in R. Can anyone help me on how I should approach this?

Answer 1

Here is the dplyr translation of your SQL code (package dplyr should be installed):

library(dplyr)

movielens_tags %>%
  right_join(movielens_ratings, by = "movieId") %>%
  group_by(tag) %>%
  summarise(mean_rating = mean(mean_rating)