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Using Semi Join on DataFrames in Julia

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In Julia, you can use the semijoin function from the DataFrames package to perform a semi-join operation on two dataframes.

A semi-join returns all rows from the first dataframe (the left dataframe) that have a match in the second dataframe (the right dataframe). The resulting dataframe will have the same number of columns as the left dataframe, and will only contain rows that have a match in the right dataframe.

Here is a visual representation of a semi-join using two dataframes df1 and df2:

df1          df2
+----+       +----+
| id |       | id |
+----+       +----+
|  1 |       |  2 |
|  2 |       |  3 |
|  3 |       |  5 |
|  4 |       +----+
+----+

Result:
+----+
| id |
+----+
|  2 |
|  3 |
+----+

Semi Join on DataFrames in Julia Examples

Here is an example of how to use the semijoin function:

using DataFrames

# Define the left dataframe
df1 = DataFrame(id = [1, 2, 3, 4], name = ["Alice", "Bob", "Charlie", "Dave"])

# Define the right dataframe
df2 = DataFrame(id = [2, 3, 5], department = ["Sales", "Marketing", "Engineering"])

# Perform the semi-join
result = semijoin(df1, df2, on = :id)

# Print the resulting dataframe
println(result)

The output of this code will be:

2×2 DataFrame
 Row │ id     name    
     │ Int64  String  
─────┼────────────────
   1 │     2  Bob
   2 │     3  Charlie

In this example, the semijoin function performs a semi-join on df1 and df2 using the id column as the join key. The resulting dataframe result contains all rows from df1 that have a matching id in df2, and only includes the id and name columns from df1.

You can also specify multiple columns as the join key by passing a vector of symbols to the on argument, like this:

result = semijoin(df1, df2, on = [:id, :name])

In this case, the semi-join will only return rows that have a matching value in both the id and name columns.

Here is another example of using the semijoin function to perform a semi-join on two dataframes:

using DataFrames

# Define the left dataframe
df1 = DataFrame(id = [1, 2, 3, 4], name = ["Alice", "Bob", "Charlie", "Dave"], department = ["Sales", "Marketing", "Engineering", "Human Resources"])

# Define the right dataframe
df2 = DataFrame(id = [2, 3, 5], department = ["Sales", "Marketing", "Engineering"])

# Perform the semi-join
result = semijoin(df1, df2, on = :department)

# Print the resulting dataframe
println(result)

The output of this code will be:

3×3 DataFrame
 Row │ id     name     department  
     │ Int64  String   String      
─────┼─────────────────────────────
   1 │     1  Alice    Sales
   2 │     2  Bob      Marketing
   3 │     3  Charlie  Engineering

In this example, the semijoin function performs a semi-join on df1 and df2 using the department column as the join key. The resulting dataframe result contains all rows from df1 that have a matching department in df2, and includes all columns from df1.

Related:

  1. Using Inner Join on DataFrames in Julia
  2. Using Right Join on DataFrames in Julia
  3. Using Left Join on DataFrames in Julia
  4. Using Outer Join on DataFrames in Julia