R Error in merge – fix.by must specify uniquely valid column (2 Examples)

This article illustrates how to handle the “Error in fix.by(by.y, y) : ‘by’ must specify a uniquely valid column” in R programming.

Construction of Example Data

my_df1 <- data.frame(ID_a = 101:106,    # Construct first example data
                     a = 1:6)
my_df1                                  # Show data in RStudio console
#   ID_a a
# 1  101 1
# 2  102 2
# 3  103 3
# 4  104 4
# 5  105 5
# 6  106 6
my_df2 <- data.frame(ID_b = 101:106,    # Construct second example data
                     b = LETTERS[1:6])
my_df2                                  # Show data in RStudio console
#   ID_b b
# 1  101 A
# 2  102 B
# 3  103 C
# 4  104 D
# 5  105 E
# 6  106 F

Example 1: Replicating the Error in fix.by(by.y, y) : ‘by’ must specify a uniquely valid column

merge(my_df1,                           # merge() function leads to error
      my_df2,
      by.x = "ID_a")
# Error in fix.by(by.y, y) : 'by' must specify a uniquely valid column

Example 2: Solving the Error in fix.by(by.y, y) : ‘by’ must specify a uniquely valid column

merge(my_df1,                           # merge() function works fine
      my_df2,
      by.x = "ID_a",
      by.y = "ID_b")

Related Articles & Further Resources

Have a look at the following list of R tutorials. They focus on topics such as merging and coding errors:

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