To find duplicate rows in SQL, first decide which columns define a duplicate. Group by those columns and use HAVING COUNT(*) > 1 to report repeated values. To remove redundant stored records while keeping one, rank each group with ROW_NUMBER(), preview the rows to be removed, and delete only the extra ones. The examples below use PostgreSQL; check your database’s syntax before running a deletion.
Choose what counts as a duplicate
A duplicate is defined by the values that matter to your task, not automatically by every column in a table. If two customer records share an email but have different names or IDs, grouping by email finds a repeated email. Grouping by every column instead finds rows whose values match across all those columns.
Decide which record should survive before deleting anything. A retention rule might be the lowest ID, the newest timestamp, or a reviewed choice. The examples use the lowest ID as a simple rule; replace it with a rule that fits your data.
Find repeated values with GROUP BY
To list customer emails that occur more than once, group by the email and filter groups with HAVING:
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SELECT email, COUNT(*) AS row_count
FROM customers
GROUP BY email
HAVING COUNT(*) > 1;This returns one result for each repeated email and its number of table rows. It does not return every underlying customer record. PostgreSQL describes GROUP BY as grouping rows that have the same values in the listed columns; see PostgreSQL’s table expressions documentation.
To find full-row duplicates, list every column whose values must match in the grouping. For a business-defined duplicate, include only the key columns that establish that definition.
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Remove duplicate output rows without changing stored data
If you only want a query result with repeated rows omitted, use SELECT DISTINCT:
SELECT DISTINCT column_a, column_b, column_c
FROM some_table;This returns unique combinations of the selected output columns; it does not delete records from some_table. PostgreSQL’s SELECT documentation defines DISTINCT as eliminating duplicate rows from the result: PostgreSQL SELECT reference.
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Preview stored duplicates and choose a survivor in PostgreSQL
To identify individual records within each duplicate-key group, use ROW_NUMBER() with a partition over the key columns. The following query previews rows and assigns rank 1 to the row with the lowest ID in each group:
SELECT id, column_a, column_b,
ROW_NUMBER() OVER (
PARTITION BY column_a, column_b
ORDER BY id
) AS row_num
FROM some_table;Replace column_a and column_b with the columns that define a duplicate. Rows numbered 2 or higher are candidates for removal under this retention rule. Inspect the output before proceeding.
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Include a unique tie-breaker in the ordering, such as a unique ID. If ordering values tie, PostgreSQL documents that tied rows receive row numbers in an unspecified order. Also, PostgreSQL permits window functions only in the SELECT list and ORDER BY, so filtering by a computed row number requires an outer query layer. See the PostgreSQL window functions documentation.
Delete extra stored rows carefully
For PostgreSQL, the general approach is to put the ranking query inside a subquery and delete rows whose rank is greater than 1. Adapt this illustrative pattern to your table, key columns, and retention rule, then validate the candidate rows before executing a destructive statement:
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DELETE FROM some_table
WHERE id IN (
SELECT id
FROM (
SELECT id,
ROW_NUMBER() OVER (
PARTITION BY column_a, column_b
ORDER BY id
) AS row_num
FROM some_table
) AS ranked
WHERE row_num > 1
);This pattern assumes id uniquely identifies a row and that the lowest ID is the one to retain. The PostgreSQL Wiki illustrates this nested-query and row_number() approach; its example is community guidance, not a guarantee of portable syntax: PostgreSQL Wiki: deleting duplicates.
- Confirm the duplicate key and survivor rule before deletion.
- Run the corresponding ranking query and inspect every row with a rank above 1.
- Use the backup, review, and transaction safeguards appropriate to your database environment.
- Check the exact syntax and behavior for your database engine and version before adapting this PostgreSQL example.
Do not omit the deletion filter: PostgreSQL states that a DELETE without a WHERE clause deletes all rows in the table. See the PostgreSQL DELETE reference.
Pick the right method
| Goal | Method | Changes stored data? | Survivor rule |
|---|---|---|---|
| Show one copy of each selected output row | SELECT DISTINCT |
No | Not applicable; repeated result rows are omitted |
| Identify repeated key groups | GROUP BY with HAVING COUNT(*) > 1 |
No | Not applicable; reports groups and counts |
| Remove redundant stored records | Rank by key with ROW_NUMBER(), then delete rows ranked above 1 |
Yes | Set explicitly with the window ORDER BY |
The deletion pattern here is PostgreSQL-specific. The cited references do not establish equivalent executable deletion syntax for SQL Server, MySQL, SQLite, or other engines; consult the target engine’s documentation before using a cleanup query.
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