What is data duplication in Snowflake?

Published
July 5, 2024
Author

Data duplication in Snowflake refers to the occurrence of identical records within a dataset. This can happen due to various reasons such as human error, system glitches, or inadequate data integration processes. Understanding and managing data duplication is crucial for maintaining data integrity and ensuring accurate data analysis.

CREATE OR REPLACE TABLE STUDENT_RECORD (
STUDENT_ID NUMBER(6,0),
FIRST_NAME VARCHAR2(20),
LAST_NAME VARCHAR2(20),
AGE NUMBER(3,0),
ADDRESS VARCHAR2(100),
PHONE_NUMBER VARCHAR2(20),
GRADE VARCHAR2(10)
);

INSERT INTO STUDENT_RECORD(STUDENT_ID, FIRST_NAME, LAST_NAME, AGE, ADDRESS, PHONE_NUMBER, GRADE) VALUES
(1, 'John', 'Cena', 18, '123 Main St, City', '123-456-7890', 'A'),
(1, 'John', 'Cena', 18, '123 Main St, City', '123-456-7890', 'A'),
(2, 'Rock', 'Bottom', 17, '456 Second St, Town', '987-654-3210', 'B'),
(2, 'Rock', 'Bottom', 17, '456 Second St, Town', '987-654-3210', 'B'),
(2, 'Rock', 'Bottom', 17, '456 Second St, Town', '987-654-3210', 'B'),
(3, 'James', 'Johnson', 16, '789 Oak St, Village', '456-123-7890', 'C'),
(3, 'James', 'Johnson', 16, '789 Oak St, Village', '456-123-7890', 'C'),
(4, 'Sarah', 'Williams', 18, '321 Pine St, County', '789-123-4560', 'A');

This code creates a sample table named STUDENT_RECORD and inserts dummy data, including duplicate rows. This setup is essential for demonstrating how to identify and delete duplicate records in Snowflake.

How can you identify and delete duplicate rows in Snowflake?

Identifying and deleting duplicate rows in Snowflake can be achieved using various methods. One common approach is to use the DISTINCT keyword, which filters out duplicate records. Another method involves using the ROW_NUMBER() window function to assign unique identifiers to rows and then delete duplicates based on these identifiers.

  • DISTINCT Keyword: The DISTINCT keyword is used with the SELECT command to return only unique records from a table, effectively removing duplicates.
  • ROW_NUMBER() Function: This window function assigns a unique sequential integer to rows within a partition of a result set, which can be used to identify and remove duplicates.
  • CTE and DELETE: A Common Table Expression (CTE) can be used to store duplicate records temporarily, which are then deleted from the main table.

Step-by-Step Process to Delete Duplicate Rows

1. Create a Demo Table

Set up a demo table and insert elements to identify duplicates.

CREATE OR REPLACE TABLE STUDENT_RECORD (
STUDENT_ID NUMBER(6,0),
FIRST_NAME VARCHAR2(20),
LAST_NAME VARCHAR2(20),
AGE NUMBER(3,0),
ADDRESS VARCHAR2(100),
PHONE_NUMBER VARCHAR2(20),
GRADE VARCHAR2(10)
);

INSERT INTO STUDENT_RECORD(STUDENT_ID, FIRST_NAME, LAST_NAME, AGE, ADDRESS, PHONE_NUMBER, GRADE) VALUES
(1, 'John', 'Cena', 18, '123 Main St, City', '123-456-7890', 'A'),
(1, 'John', 'Cena', 18, '123 Main St, City', '123-456-7890', 'A'),
(2, 'Rock', 'Bottom', 17, '456 Second St, Town', '987-654-3210', 'B'),
(2, 'Rock', 'Bottom', 17, '456 Second St, Town', '987-654-3210', 'B'),
(2, 'Rock', 'Bottom', 17, '456 Second St, Town', '987-654-3210', 'B'),
(3, 'James', 'Johnson', 16, '789 Oak St, Village', '456-123-7890', 'C'),
(3, 'James', 'Johnson', 16, '789 Oak St, Village', '456-123-7890', 'C'),
(4, 'Sarah', 'Williams', 18, '321 Pine St, County', '789-123-4560', 'A');

This code creates a sample table named STUDENT_RECORD and inserts dummy data, including duplicate rows.

2. Store Duplicate Records

Use a query to create a temporary table that stores all duplicate records.

WITH DUPLICATES AS (
SELECT STUDENT_ID, FIRST_NAME, LAST_NAME, AGE, ADDRESS, PHONE_NUMBER, GRADE,
ROW_NUMBER() OVER(PARTITION BY STUDENT_ID ORDER BY STUDENT_ID) AS RANK
FROM STUDENT_RECORD
)
DELETE FROM STUDENT_RECORD WHERE STUDENT_ID IN (
SELECT STUDENT_ID FROM DUPLICATES WHERE RANK > 1
);

This query uses a Common Table Expression (CTE) to identify and store duplicate records based on the ROW_NUMBER() function.

3. Reinsert Unique Records

Reinsert the unique rows from the temporary table back into the main table.

INSERT INTO STUDENT_RECORD (STUDENT_ID, FIRST_NAME, LAST_NAME, AGE, ADDRESS, PHONE_NUMBER, GRADE)
SELECT STUDENT_ID, FIRST_NAME, LAST_NAME, AGE, ADDRESS, PHONE_NUMBER, GRADE
FROM STUDENT_RECORD
WHERE RANK = 1;

This code reinserts the unique records back into the STUDENT_RECORD table, ensuring that only unique rows are retained.

Common Challenges and Solutions

  • Ensuring Data Integrity: Always back up your data before performing deletion operations to prevent accidental data loss.
  • Handling Large Datasets: For large datasets, consider using more efficient methods like the ROW_NUMBER() function to minimize performance impact.
  • Verifying Results: After deleting duplicates, verify the results by running a SELECT query to ensure that only unique records remain.

Recap

  • Data duplication in Snowflake can negatively impact data quality and analysis.
  • Various methods, such as using DISTINCT, ROW_NUMBER(), and CTEs, can effectively identify and delete duplicate records.
  • Ensuring data integrity requires a well-structured approach to handle duplicates, considering both the causes and impacts of data duplication.

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