Finding duplicate values in a SQL table

Finding duplicate values in a SQL table is a common task when you want to clean up data, enforce uniqueness, or analyze repeated records. But how exactly can you identify duplicates based on one or more columns?

How to find duplicates in SQL?

The most common approach is to use the GROUP BY clause combined with HAVING. This lets you group rows by the columns you want to check and then filter groups having more than one record.

Example query to find duplicates:

SELECT column1, column2, COUNT(*)

FROM your_table

GROUP BY column1, column2

HAVING COUNT(*) > 1;

• Here, column1 and column2 represent the columns you want to check for duplicates.
  
• COUNT(*) counts how many times each combination appears.
  
• The HAVING COUNT(*) > 1 filters the results to show only duplicates.
  

Key points to remember:

• You can check duplicates on a single column or multiple columns by adjusting the GROUP BY clause.
  
• This method helps identify the exact values that are duplicated.
  
• To see the full rows of duplicates, you can join this result back to the original table.
  
• Bonus: Finding duplicate rows with all columns
  

If you want to find rows that are completely identical across all columns, you can group by all columns like this:

SELECT *, COUNT(*)

FROM your_table

GROUP BY column1, column2, column3, ... -- all columns here

HAVING COUNT(*) > 1;

Summary:

• Use GROUP BY with HAVING COUNT(*) > 1 to spot duplicates.
  
• Adjust columns in the GROUP BY clause based on your needs.
  
• This approach is efficient and widely supported by SQL databases.