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SQL Data Cleaning for Handling Missing Values and Duplicates


Data cleaning is a crucial step in the data preprocessing pipeline, ensuring that the data used for analysis or machine learning is accurate and reliable. In this article, we will focus on strategies for handling missing values and duplicates in SQL databases, providing theoretical insights into best practices.

SQL Data Cleaning for Handling Missing Values and Duplicates

I. Dealing with Missing Values:

Understanding Missing Values:
  • Null values: Null or missing values can occur for various reasons, such as data entry errors, sensor malfunctions, or incomplete records.

Identifying Missing Values:
  • SQL functions: Use functions like COUNT, IS NULL, or IS NOT NULL to identify the presence of missing values in columns.

Imputation Techniques:
  • Mean, Median, Mode: Replace missing values with the mean, median, or mode of the respective column.

  • Forward or Backward Fill: Use the values from the previous or next row to fill missing values.

  • Interpolation: Estimate missing values based on the values of neighboring data points.

Deletion Strategies:
  • Remove Rows: Delete rows containing missing values.

  • Remove Columns: Eliminate entire columns if a significant portion of data is missing.

II. Handling Duplicate Data:

Identifying Duplicates:
  • Duplicate Rows: Use SQL's GROUP BY and HAVING clauses to identify rows with duplicate values in specified columns.

  • Unique Constraints: Leverage unique constraints to prevent duplicates in critical columns.

Removing Duplicates:
  • DISTINCT Keyword: Use the DISTINCT keyword in SELECT queries to retrieve unique records.

  • DELETE Statement: Remove duplicate rows using the DELETE statement with the help of the ROW_NUMBER() window function.

Prevention Strategies:
  • Primary Keys: Define primary keys to ensure each record is unique.

  • Constraints: Implement unique constraints on specific columns to prevent duplicate entries.

III. Best Practices:

  • Data Profiling: Understand the data distribution and patterns before deciding on missing value imputation or duplicate removal techniques.

  • Automated Processes: Implement automated scripts or stored procedures to regularly clean and maintain data integrity.

  • Logging and Auditing: Keep logs of data cleaning processes to trace changes and identify potential issues.

  • Regular Monitoring: Continuously monitor data quality and update cleaning processes as needed, especially when new data is added.


Data cleaning is a critical aspect of maintaining data integrity and ensuring accurate analysis. By understanding and implementing effective strategies for handling missing values and duplicates in SQL databases, organizations can enhance the reliability of their data, leading to more informed decision-making processes.

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