In Julia, you can use the leftjoin
function to perform a left join on two dataframes. Here's an example of how to use leftjoin
to perform a left join:
Left Join on DataFrames in Julia Examples
using DataFrames
# Define the two dataframes to join
df1 = DataFrame(id=[1, 2, 3], name=["Alice", "Bob", "Charlie"])
df2 = DataFrame(id=[1, 2, 3, 4], score=[90, 80, 70, 60])
# Perform the left join
result = leftjoin(df1, df2, on=:id)
# Print the resulting dataframe
println(result)
This will output the following dataframe:
3×3 DataFrame
Row │ id name score
│ Int64 String Int64?
─────┼────────────────────────
1 │ 1 Alice 90
2 │ 2 Bob 80
3 │ 3 Charlie 70
Note that the score
column in the resulting dataframe has type Int64?
, which indicates that it is nullable. This is because the score
column in the df2
dataframe may contain null values, and the leftjoin
function will preserve those null values in the resulting dataframe.
You can also specify the makeunique
keyword argument to ensure that the resulting dataframe has unique rows. For example:
result = leftjoin(df1, df2, on=:id, makeunique=true)
This will remove any duplicate rows from the resulting dataframe.
Here's another example of using the leftjoin
function to perform a left join on two dataframes with multiple join keys and multiple columns:
using DataFrames
# Define the two dataframes to join
df1 = DataFrame(id=[1, 2, 3], name=["Alice", "Bob", "Charlie"], city=["New York", "Chicago", "Los Angeles"], year=[2020, 2021, 2020])
df2 = DataFrame(id=[1, 2, 3, 4], score=[90, 80, 70, 60], year=[2020, 2021, 2020, 2021])
# Perform the left join
result = leftjoin(df1, df2, on=[:id, :year])
# Print the resulting dataframe
println(result)
This will output the following dataframe:
3×5 DataFrame
Row │ id name city year score
│ Int64 String String Int64 Int64?
─────┼────────────────────────────────────────────
1 │ 1 Alice New York 2020 90
2 │ 2 Bob Chicago 2021 80
3 │ 3 Charlie Los Angeles 2020 70
Alternatively, you can modify the code like this to remove the :year
column from the on
argument in the leftjoin
function:
using DataFrames
# Define the two dataframes to join
df1 = DataFrame(id=[1, 2, 3], name=["Alice", "Bob", "Charlie"], city=["New York", "Chicago", "Los Angeles"])
df2 = DataFrame(id=[1, 2, 3, 4], score=[90, 80, 70, 60], year=[2020, 2021, 2020, 2021])
# Perform the left join
result = leftjoin(df1, df2, on=:id)
# Print the resulting dataframe
println(result)
This will output the following dataframe:
3×5 DataFrame
Row │ id name city score year
│ Int64 String String Int64? Int64?
─────┼─────────────────────────────────────────────
1 │ 1 Alice New York 90 2020
2 │ 2 Bob Chicago 80 2021
3 │ 3 Charlie Los Angeles 70 2020