How To Merge Two Dataframes in Pandas

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Merging data is an important process in data analysis and manipulation, allowing us to combine separate datasets into one for more complex and insightful investigations. This is a process often encountered when dealing with Pandas, a robust data manipulation library in Python. This tutorial will dive deep into the process of merging two DataFrames in Pandas, revealing the different techniques and scenarios in which each can be applied. Whether you are a novice data enthusiast or a seasoned data scientist, this guide will help you understand and master the art of merging dataframes in Pandas. You’ll learn about different join types, how to handle inconsistencies in your data, and how to ensure smooth data merging for efficient data analysis.

  1. What Are Pandas DataFrames
  2. Why Merge DataFrames in Pandas
  3. How To Merge DataFrames Using the Merge Function
  4. Different Types of Joins: Inner, Outer, Left, and Right
  5. Can We Merge DataFrames on Multiple Columns
  6. How To Handle Inconsistencies During DataFrame Merge
  7. Real-World Examples of Merging DataFrames in Pandas
  8. Troubleshooting Common Errors During DataFrame Merges
  9. Does The Order of DataFrames Affect the Merge Result
  10. What Are The Best Practices When Merging Large DataFrames

What Are Pandas DataFrames

Pandas DataFrames are the backbone of the Python-based data analysis library, Pandas. They can be best understood as two-dimensional labeled data structures that are capable of storing heterogeneous data types including integers, strings, and floating-point numbers. In essence, a DataFrame is akin to a spreadsheet, a SQL table, or the data.frame in R.

DataFrame characteristics:

  • Columns can be of different types: You can have integers, float, and string types all within the same DataFrame.
  • Size Mutable: The size of a DataFrame can be modified – columns and rows can be added and deleted.
  • Labeled axes: Rows and columns are labeled, enabling easier data manipulation.
  • Arithmetic operations on rows and columns: You can perform mathematical operations on rows and columns, including aggregation functions like sum, average, etc.

To put it simply, imagine this sample DataFrame:


Here, the ‘Index’ provides labels to the rows, and ‘Column1’ and ‘Column2’ are column labels. ‘A’, ‘B’, ‘C’ and 1, 2, 3 are the data stored within these respective rows and columns.

In the next sections, we will dive deeper into how these DataFrames interact with one another during merging, and explore different techniques to effectively merge these DataFrames.

Why Merge DataFrames in Pandas

The ability to merge DataFrames in Pandas is an essential skill for any data enthusiast or professional working with Python. But why exactly do we need to merge DataFrames?

  1. Data Combination: Often, information is spread across multiple dataframes. By merging, we can combine this data into a single DataFrame for comprehensive analysis.
  2. Data Enrichment: Merging allows us to add additional information to our DataFrame without manually entering it. This can enrich our data and offer more insightful analysis.
  3. Efficiency: Merging can help reduce the redundancy of data storage and improve the efficiency of data processing.
  4. Data Alignment: Merging helps align disparate datasets based on common identifiers or indices.

Imagine you have two DataFrames as follows:

DataFrame 1:

1John Doe
2Jane Doe
3Jim Beam

DataFrame 2:


To get a comprehensive view of each customer’s name with their respective purchases, we would need to merge these two DataFrames based on the ‘Customer_ID’ column.

How To Merge DataFrames Using the Merge Function

In Python’s Pandas library, the merge function is a powerful tool to combine two DataFrames. Let’s explore how we can use this function.

The syntax for the merge function is as follows:

pd.merge(left, right, how='inner', on=None)

Here, ‘left’ and ‘right’ are the DataFrames you want to merge. The ‘how’ argument specifies the type of merge to be performed—’inner’, ‘outer’, ‘left’, or ‘right’. The ‘on’ parameter is used to tell pandas which column or index to use as the join key.

Using the earlier example of customer DataFrames, we could merge them based on the ‘Customer_ID’ column using the merge function:

merged_df = pd.merge(df1, df2, on='Customer_ID')

This results in a new DataFrame, ‘merged_df’, as shown below:


1John Doe30
2Jane Doe50
3Jim Beam70

Now, we have a single DataFrame that combines the customer names with their respective purchases, providing a more comprehensive view of the data.

Different Types of Joins: Inner, Outer, Left, and Right

In Pandas, when we merge two DataFrames, we have four types of joins: Inner Join, Outer Join, Left Join, and Right Join. Each join type behaves differently when it encounters null values or missing keys in the joined columns.

  1. Inner Join: This is the default join in Pandas merge operation. The result includes rows for which the join-key is in both the original DataFrames.
pd.merge(df1, df2, on='key', how='inner')
  1. Outer Join (or Full Join): This join includes rows for which the join-key is in either or both original DataFrames. Any missing fields are filled with NaN values.
pd.merge(df1, df2, on='key', how='outer')
  1. Left Join: This join includes all rows from the left DataFrame and any rows with matching keys from the right DataFrame. If there is no match, the right side will contain NaN.
pd.merge(df1, df2, on='key', how='left')
  1. Right Join: This join includes all rows from the right DataFrame and any rows with matching keys from the left DataFrame. If there is no match, the left side will contain NaN.
pd.merge(df1, df2, on='key', how='right')

Understanding these different types of joins helps you to select the appropriate method for your data merging needs. The next sections will walk through examples and how to handle potential inconsistencies during merging.

Can We Merge DataFrames on Multiple Columns

Absolutely! Pandas provides the flexibility to merge DataFrames on multiple columns. This feature is particularly useful when a single column does not uniquely identify rows in your data, and a combination of columns is required.

To merge on multiple columns, we just need to pass a list of column names to the ‘on’ parameter in the merge function:

merged_df = pd.merge(df1, df2, on=['column1', 'column2'])

Let’s assume we have two DataFrames:

DataFrame 1:


DataFrame 2:


Now, if we want to merge these two DataFrames based on both ‘Product_ID’ and ‘Location’, we could do:

merged_df = pd.merge(df1, df2, on=['Product_ID', 'Location'])

This results in a new DataFrame:



Notice that the merged DataFrame only contains rows with matching ‘Product_ID’ and ‘Location’ in both original DataFrames.

In the following sections, we will continue exploring other aspects of merging, such as handling inconsistencies during DataFrame merge.

How To Handle Inconsistencies During DataFrame Merge

Dealing with inconsistencies is a common part of data analysis. During a DataFrame merge, inconsistencies might appear in various forms – missing values, mismatched data types, or duplicates. Here are some strategies on how to handle inconsistencies during DataFrame merge:

  1. Missing Values: After merging, if certain fields are missing (represented by NaN), you can handle these using methods like fillna(), dropna(), or data imputation.
merged_df.fillna('Unknown')  # Replace NaNs with 'Unknown'
  1. Mismatched Data Types: Before merging, ensure that the columns you’re merging on have the same data type in both DataFrames. You can use the astype() function to convert data types.
df['column_name'] = df['column_name'].astype('desired_type')
  1. Duplicates: Duplicates can cause issues in the merged DataFrame. Use drop_duplicates() before or after merging to handle this.
merged_df = merged_df.drop_duplicates()
  1. Inconsistent Naming: If the same column has a different name in each DataFrame, use the left_on and right_on parameters in the merge() function.
merged_df = pd.merge(df1, df2, left_on='column1_df1', right_on='column1_df2')

By applying these strategies, you can mitigate inconsistencies and ensure accurate DataFrame merging. In the next sections, we’ll examine real-world examples of merging DataFrames in Pandas and tackle some common errors.

Real-World Examples of Merging DataFrames in Pandas

Using Pandas DataFrame merge function can be quite handy in real-world applications. Here are some examples that demonstrate its power and versatility:

  1. E-commerce Analytics: Suppose you have one DataFrame that holds customer information (name, email, etc.) and another DataFrame with transaction details (purchase amount, product ID, etc.). By merging these two DataFrames on the common ‘customer_ID’ column, you can get a comprehensive view of each customer’s purchasing behavior.
customer_transactions = pd.merge(customers_df, transactions_df, on='customer_ID')
  1. Healthcare Data Analysis: Consider a case where one DataFrame contains patient demographic data (age, gender, location), and another DataFrame contains medical history (disease, treatment). You can merge these two DataFrames on ‘patient_ID’ to gain a holistic view of each patient’s profile.
patient_data = pd.merge(demographics_df, history_df, on='patient_ID')
  1. Financial Data Aggregation: Imagine you have separate DataFrames for company details (name, sector, location) and financial data (revenue, expenses). You can merge these DataFrames on ‘company_ID’ for a combined view, enabling more thorough financial analysis.
financial_overview = pd.merge(company_df, financial_df, on='company_ID')

In each of these examples, merging allows us to combine data from different sources into one cohesive DataFrame, facilitating more complex and detailed analysis. In the next section, we’ll look at troubleshooting common errors during DataFrame merges.

Troubleshooting Common Errors During DataFrame Merges

Here are some common issues and their solutions:

  1. Key Error: This occurs if the key/column you are trying to merge on does not exist in one or both DataFrames. Check the column names in your DataFrame using df.columns.
  2. Memory Error: If your DataFrames are too large, merging them might exceed your system’s memory. Consider breaking your operation into chunks, or using the on parameter to merge only on specific columns.
  3. Type Error: This happens if the columns you are merging on are of different types in the two DataFrames. Use the astype() function to make the data types consistent before merging.
df['column_name'] = df['column_name'].astype('desired_type')
  1. Duplicate Values: Duplicate entries in your key column(s) can lead to unexpected results after the merge. Consider dropping duplicates using drop_duplicates() before merging.
df = df.drop_duplicates(subset='column_name')
  1. NaN values after merge: If you find NaN values after a left/right join, it means those values do not exist in the other DataFrame. You can handle NaN values using fillna() or dropna().
merged_df = merged_df.fillna('Unknown')

These solutions will help you to troubleshoot common errors and ensure a smoother merging process.

Does The Order of DataFrames Affect the Merge Result

The order of DataFrames in a merge operation does indeed affect the outcome, particularly when we’re dealing with left and right joins. The terms ‘left’ and ‘right’ refer to the order in which the DataFrames are specified in the merge function.

Let’s look at an example. Given two DataFrames, df1 and df2, if we perform a left join with df1 as the left DataFrame:

merged_df = pd.merge(df1, df2, on='key', how='left')

This operation will keep all rows from df1 (the left DataFrame) and add matching rows from df2 (the right DataFrame) where available. If there is no match, the result is NaN.

On the other hand, if we reverse the order and perform a left join with df2 as the left DataFrame:

merged_df = pd.merge(df2, df1, on='key', how='left')

Now, all rows from df2 are kept, and matching rows from df1 are added where available.

As such, the order of the DataFrames in the merge function call directly impacts the resulting DataFrame in case of left and right joins.

However, for inner and outer joins, the order of DataFrames does not affect the result, as these operations are symmetric. They yield the same result irrespective of which DataFrame is on the left or right.

What Are The Best Practices When Merging Large DataFrames

When dealing with large DataFrames, the process of merging can become challenging due to increased computational requirements and potential memory issues. Here are some best practices to consider:

  1. Pre-Filter DataFrames: Before merging, filter out unnecessary rows or columns that are not required for analysis. This can significantly reduce the size of the DataFrame and improve performance.
df = df[['column1', 'column2']]  # Keep only necessary columns
  1. Use Appropriate Data Types: Columns with smaller data types consume less memory. If possible, convert columns to categories, integer or boolean types.
df['column_name'] = df['column_name'].astype('category')
  1. Break Down the Task: If your DataFrames are too large to be merged at once, consider breaking down the operation into chunks.
  2. Use ‘on’ Parameter Wisely: While merging, merge on the most specific column(s) possible using the ‘on’ parameter. This reduces the computation time and makes the merge operation more efficient.
merged_df = pd.merge(df1, df2, on='specific_column')
  1. Drop Duplicates: Duplicates can significantly slow down the merging process. Consider dropping duplicates before merging.
df = df.drop_duplicates()
  1. Use In-Place Operations: Some pandas operations have an inplace parameter. Setting inplace=True can sometimes save memory by avoiding creating copies of DataFrames.

Following these best practices will help you to effectively manage memory usage and improve the performance when merging large DataFrames.

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