How can I find duplicate records in a table?

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If you’re often working with large data sets, you might at times need to identify and manipulate duplicate records. SQL, with its built-in functions, can be a powerful tool to help you find these duplicates. Let’s explore how.

Understanding Duplicate Records

Firstly, it’s necessary to clarify what we mean by “duplicates”. In the context of a database, a duplicate row is defined as another row that has exactly the same values in each column as the original row.

Finding Duplicate Records

This SQL statement identifies duplicates by aggregating identical rows and counting grouped rows. If the count is more than 1, the row is a duplicate. However, this will only return the duplicated data; it will not tell you the specific rows that are duplicated in the original table.

Example:

Suppose you have a ‘Users’ table with ‘UserID’ and ‘UserEmail’. To find duplicate email addresses, you would use:

This query will return email addresses that exist more than once in the ‘Users’ table.

Detecting Duplicate Records with the ROW_NUMBER Function

If you need more information, such as the complete duplicated rows or the original IDs, the ROW_NUMBER window function can come in handy.

This code uses a subquery to partition data by the column_name and assigns a row number to each partition. Duplicate records will have a RN value greater than 1.

Example:

Same ‘Users’ table, to find complete duplicate records:

This will retrieve each row where the ‘UserEmail’ appears more than once.

Wrap-Up

Finding duplicate values is a common task when you’re manipulating data. SQL, with its analytic functions, offers efficient ways to find these duplicates. Remember, it’s important to know your data and understand the context to choose the right approach.