13 Joining

(AST230) R for Data Science

Author

Md Rasel Biswas

Joins

It’s rare that a data analysis involves only a single data frame.
Typically you have many data frames, and you must join them together to answer the questions that you’re interested in.

Motivational example

Year 1

# A tibble: 7 × 4
     id gender ast101 ast102
  <int> <chr>   <dbl>  <dbl>
1   101 F          57     49
2   102 M          51     51
3   103 F          72     26
4   104 F          58     58
5   105 M          65     32
6   106 M          57     62
7   107 F          65     66

Year 2

# A tibble: 7 × 3
     id ast201 ast202
  <dbl>  <dbl>  <dbl>
1   101     77     43
2   102     72     34
3   103     65     41
4   104     76     39
5   105     75     37
6   106     70     35
7   201     76     65

Create a variable grade which takes the following values:
- 4.0 (for score \geq 80), 3.75 (for 75\leq score <80),
- 3.5 (for 70\leq score <75), 3.25 (for 65\leq score <70),
- 3.0 (for 60\leq score <65), , 2.5 (for 50\leq score <65), and
- 0 (for score <50)

Suppose, ast101 and ast201 are 4-credit course and other courses are of three credits
Calculate the GPA for each student for two years separately
Calculate CGPA, i.e., overall performance of each student
Compare the performance of male and female on the basis of CGPA

Joins

dplyr provides six join functions:
- left_join(), inner_join(), right_join(), and full_join()
- semi_join(), and anti_join()
They all have the same interface:
- they take a pair of data frames (x and y) and return a data frame

Mutating joins

A mutating join allows you to combine variables from two data frames:
- it first matches observations by their keys, then copies across variables from one data frame to the other
- Like mutate(), the join functions add variables to the right
There are four types of mutating join
- left_join(), inner_join(), right_join(), full_join()

Let’s define two simple tibbles x and y.

x <- tribble(
  ~key, ~val_x,
     1, "x1",
     2, "x2",
     3, "x3"
)
y <- tribble(
  ~key, ~val_y,
     1, "y1",
     2, "y2",
     4, "y3"
)

To understand how joins work, it’s useful to think of every possible match.
Here we show that with a grid of connecting lines

Inner join

xy <- inner_join(x = x, y = y, by = "key")

xy

# A tibble: 2 × 3
    key val_x val_y
  <dbl> <chr> <chr>
1     1 x1    y1   
2     2 x2    y2

xy_left <- left_join(x = x, y = y, by = "key")

xy_left

# A tibble: 3 × 3
    key val_x val_y
  <dbl> <chr> <chr>
1     1 x1    y1   
2     2 x2    y2   
3     3 x3    <NA>

xy_right <- right_join(x = x, y = y,
                    by = "key")

xy_right

# A tibble: 3 × 3
    key val_x val_y
  <dbl> <chr> <chr>
1     1 x1    y1   
2     2 x2    y2   
3     4 <NA>  y3

xy_full <- full_join(x = x, y = y, by = "key")

xy_full

# A tibble: 4 × 3
    key val_x val_y
  <dbl> <chr> <chr>
1     1 x1    y1   
2     2 x2    y2   
3     3 x3    <NA> 
4     4 <NA>  y3

The following Venn diagrams showing the difference between inner, left, right, and full joins.

Filtering joins

Mutating joins add columns from y to x, matching rows based on the key.
Filtering joins filter rows from x based on the presence or absence of matches in y.
Two types of filtering join:
- semi_join(), and anti_join()

semi_join() return all rows from x with a match in y

anti_join() keeps rows in x that match zero rows in y

Exam data

Year 1

# A tibble: 7 × 4
     id gender ast101 ast102
  <int> <chr>   <dbl>  <dbl>
1   101 F          57     49
2   102 M          51     51
3   103 F          72     26
4   104 F          58     58
5   105 M          65     32
6   106 M          57     62
7   107 F          65     66

Year 2

# A tibble: 7 × 3
     id ast201 ast202
  <dbl>  <dbl>  <dbl>
1   101     77     43
2   102     72     34
3   103     65     41
4   104     76     39
5   105     75     37
6   106     70     35
7   201     76     65

Calculate CGPA, i.e., overall performance of each student
Compare the performance of male and female on the basis of CGPA