5 Ways to Find Duplicate Data in SQL

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If you are a beginner this is the most common SQL interview question "How to find duplicate data ?" Here are 5 different ways

{ Using GROUP BY & HAVING Clause } • This method is effective for finding duplicate records based on specific columns • Group the data by the columns you want to check for duplicates • Use the HAVING clause to filter groups with more than one record

{ Using Self Join } • A self-join can be used to compare rows within the same table • Join the table with itself based on the columns you want to check for duplicates • Filter the results to find rows where the primary key values differ but the other columns are identical

{ Using ROW_NUMBRE() } • This method assigns a sequential number to each row within a partition • Create a partitioned result set using ROW_NUMBER() • Filter rows with a ROW_NUMBER greater than 1 to identify duplicates

{ Using EXISTS Clause } • The EXISTS clause can be used to check for duplicate records based on certain conditions • Create a subquery to find potential duplicate records • Use the EXISTS clause to check if a matching record exists in the subquery

{ Using DISTINCT } • While not directly for finding duplicates, DISTINCT can be used to identify unique values in a column • Count the total number of rows • Count the number of distinct values in the target column • If the counts differ, duplicates exist