Solved: pandas join non-unique

Pandas is a widely used Python library in the field of data manipulation and analysis. It provides data structures and functions needed to work with structured data seamlessly. One of the many features it offers is the ability to join tables with non-unique keys, which can be a common requirement in practical applications. In this article, we will dive into the solution to this problem, explore the step-by-step explanation of the code used for joining pandas DataFrame objects with non-unique keys, and discuss the libraries and functions involved in this process.


Joining tables is a fundamental operation performed in data manipulation and analysis tasks. In certain scenarios, we may be required to join tables on a non-unique key, which can present challenges. However, working with the powerful Python library, pandas, allows us to elegantly solve this problem using its flexible functionality.

Joining Pandas DataFrames with Non-Unique Keys

To join DataFrames in pandas, we can use the `merge()` function, which supports joining on non-unique keys. However, it is essential to understand that the result of joining non-unique keys may be different than expected, as it can lead to a cartesian product, potentially resulting in a significant increase in the number of rows in the resulting DataFrame.

Here is the step-by-step guide to using the `merge()` function to join DataFrames with non-unique keys:

import pandas as pd

# Create sample DataFrames
df1 = pd.DataFrame({"key": ["A", "B", "A", "C"], "value": [1, 2, 3, 4]})
df2 = pd.DataFrame({"key": ["A", "B", "A", "D"], "value2": [5, 6, 7, 8]})

# Perform the merge operation
result = df1.merge(df2, on="key", how="inner")

In the example above, we first import the pandas library and create two sample DataFrames (df1 and df2). Then, we use the `merge()` function to join the DataFrames on the “key” column, which contains non-unique values (A and B are repeated). The `how` parameter is set to “inner”, as we want to keep only rows that have matching keys in both DataFrames.

Understanding the Pandas Merge Function

The `merge()` function in pandas is a very powerful and flexible tool to perform table join operations. In addition to joining the DataFrames with non-unique keys, it supports various levels of customization, allowing you to have full control over the resulting DataFrame.

The `merge()` function has several important parameters such as:

  • left and right: These are the DataFrames to be merged.
  • on: The column(s) that should be used for joining the DataFrames. This can be a single column name or a list of column names when joining on multiple columns.
  • how: It defines the type of join to be performed. The options include ‘left’, ‘right’, ‘outer’, and ‘inner’. The default is ‘inner’.
  • suffixes: This is a tuple of string suffixes to apply to the overlapping columns. The default suffix is _x for the left DataFrame and _y for the right DataFrame.

These parameters can be tweaked as per your needs to perform various types of join operations and customize the output.

Similar Functions in Pandas

Apart from the `merge()` function, pandas also offers other functions for combining DataFrames in different ways, such as:

  • concat(): This function is used to concatenate DataFrames along a particular axis. You can control the concatenation by specifying various parameters such as axis, join, and keys.
  • join(): This is a convenient method available on DataFrame objects to perform join operations. It is essentially a wrapper around the merge() function, with the left DataFrame being assumed as the caller DataFrame.

In conclusion, by using the pandas `merge()` function, you can easily join DataFrames with non-unique keys. The rich set of parameters available in the `merge()` function offers full control over the joining process, catering to various data manipulation requirements. The pandas library continues to be an indispensable tool for data analysts and it offers various other functions to combine and manipulate DataFrames efficiently.

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