How to Find Duplicate Rows Based on Multiple Fields in a SQL Table?

Locating Duplicate Records Using Multiple Fields in SQL

Identifying duplicate entries based on a single column is a simple SQL task. For example, to find duplicate emails in a users table:

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

The complexity increases when identifying duplicates across multiple fields, such as email and name.

To pinpoint rows with identical email and name combinations, use this query:

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

This groups the data by both name and email, then filters to show only those groups with more than one entry, thus revealing the duplicates.

How it Works:

The HAVING clause is crucial; it filters the grouped results, ensuring only those combinations of name and email appearing more than once are returned. A typical output would resemble:

<code>| name | email       | COUNT(*) |
|------|-------------|----------|
| Tom   | john@example.com | 2        |
| Tom   | tom@example.com  | 2        |</code>

Important Consideration:

Database systems vary in their handling of grouping non-aggregated columns. Some might require explicit inclusion in the GROUP BY clause. If your database system doesn't support this implicit grouping, you'll need to adjust the query accordingly.

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