What techniques are used for data deduplication in ETL?

Data deduplication in ETL processes ensures data quality by identifying and removing duplicate records. Three common techniques include using unique identifiers, applying fuzzy matching algorithms, and leveraging SQL windowing functions. Each method addresses different scenarios, from exact matches to near-duplicates, and balances accuracy with computational efficiency.

The first technique involves unique identifiers and hashing. Unique keys, such as customer IDs or transaction numbers, are used to detect exact duplicates. For example, if a customer record is loaded twice with the same ID, the ETL process can skip or overwrite the duplicate. When unique keys aren’t available, hashing combines multiple fields (e.g., name, email, phone) into a single hash value (using MD5 or SHA-256). Comparing hashes instead of raw data speeds up detection. For instance, hashing “John Doe, johndoe@email.com, 555-1234” generates a unique string that can be checked against existing records. This method is efficient for structured data but may miss duplicates if fields have minor variations, like typos or formatting differences.

Fuzzy matching and blocking handle inexact duplicates. Algorithms like Levenshtein distance or Jaro-Winkler measure similarity between strings. For example, “Jon Smith” and “John Smith” might be flagged as potential duplicates. To reduce computational load, blocking groups records into subsets (e.g., by ZIP code or first three letters of a name) before applying fuzzy matching. Tools like Apache Spark’s MLlib provide built-in functions for these operations. For instance, a healthcare ETL pipeline might block patient records by birth year and then compare addresses and names within each block. While effective, fuzzy matching requires careful tuning of similarity thresholds to balance false positives and missed duplicates.

SQL windowing functions and incremental deduplication are useful in batch processing. Functions like ROW_NUMBER() rank duplicates within partitions, allowing developers to keep the first or most complete record. For example, ROW_NUMBER() OVER (PARTITION BY email ORDER BY last_updated DESC) assigns a rank to each duplicate email, letting the ETL process retain only the latest entry. Incremental deduplication focuses on new data by comparing it against existing records using change data capture (CDC) or timestamps. Tools like Talend or Informatica automate these steps, reducing redundant checks. For instance, a daily sales feed might only compare today’s entries against the past week’s data. This approach minimizes processing time but depends on reliable timestamp or CDC metadata.

These techniques can be combined for robust deduplication. For example, exact matching with hashes might handle 80% of duplicates, while fuzzy matching addresses edge cases. Developers should profile data upfront to choose the right mix of methods and validate results through sampling or automated tests.