How Can I Find Duplicate Rows Across Multiple Columns in a SQL Table?

Find duplicate values ​​in multiple columns in SQL table

The standard approach using the GROUP BY clause effectively identifies duplicate values ​​in a single column. But what if you need to find duplicates in multiple columns, such as name and email?

To fix this problem, just extend the GROUP BY clause to include all relevant columns:

<code class="language-sql">SELECT
    name,
    email,
    COUNT(*)
FROM
    users
GROUP BY
    name, 
    email
HAVING 
    COUNT(*) > 1</code>

By grouping by name and email, this query will effectively identify duplicates for these two column combinations.

Note: Different database systems may have different support for this syntax. Some older versions of SQL databases require all non-aggregated columns to be included in the GROUP BY clause. However, modern database engines such as PostgreSQL and MySQL often support grouping on non-aggregated columns.

Remember that functional dependency rules apply, meaning the columns used for grouping should have a strong relationship with each other. If the relationship is weak, you may get unexpected results.

To ensure consistent behavior across different database systems, it is recommended to include all non-aggregate columns in the GROUP BY clause, or to consult the documentation for your specific database for implementation details.

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