Sunday, September 24, 2023

How do you write a SQL query to find duplicates in a table?

 To find duplicates in a table using SQL, you can use the GROUP BY clause in combination with the HAVING clause. This allows you to group rows based on a column (or columns) and then filter the groups to find those that have more than one row, indicating duplicates.

 Here's how you can write a SQL query to find duplicates in a table:

Basic Syntax:

SELECT column1, column2, ...

FROM table_name

GROUP BY column1, column2, ...



  • column1, column2, ...: List the columns you want to check for duplicates.
  • table_name: The name of the table in which you want to find duplicates.
  • GROUP BY: Group the rows based on the specified columns.
  • HAVING COUNT(*) > 1: Filter the groups to include only those with more than one row.


Suppose you have a table named "employees," and you want to find duplicate email addresses:

      SELECT email

      FROM employees

      GROUP BY email

      HAVING COUNT(*) > 1;

In this example:

  • We're selecting the "email" column from the "employees" table.
  • We group the rows by the "email" column, which means that rows with the same email address are grouped together.
  • The HAVING COUNT(*) > 1 condition ensures that only groups with more than one row (i.e., duplicate email addresses) are included in the result.

The result of this query will be a list of email addresses that appear more than once in the "employees" table, indicating the presence of duplicates.

Keep in mind that the specific columns you choose for checking duplicates depend on your data and requirements. You can use the same approach to find duplicates based on multiple columns by including all relevant columns in the GROUP BY clause.

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