Although SQL provides us with constraints to prevent duplicate data, you may encounter an already existing database with duplicate records.
Using this tutorial, you will discover how to identify duplicate rows within a database.
SQL Find Duplicate
The first method we can use to find duplicate rows is the count function.
Assume we have a table with sample data as shown:
id serial,
product_name VARCHAR(255),
quantity INT
);
INSERT INTO products(product_name, quantity)
VALUES ('apples', 100),
('oranges', 120),
('apples', 100),
('bananas', 300),
('oranges', 100),
('bananas', 300);
The above query should return the table as shown:
To filter for duplicate records, we can use the query as shown below:
QUANTITY
FROM PRODUCTS
GROUP BY PRODUCT_NAME,
QUANTITY
HAVING COUNT(ID) > 1;
The query above uses the group by and count function to search for duplicate records. This should return the output as shown:
The above query works by creating a group of rows with the same values. This is accomplished using the group by clause. We then find out which of the groups has a count greater than 1. This means that there are duplicates in the group.
Terminating
In this article, you discovered how to find duplicate records in SQL using the group by and count clause.
Thanks for reading!!
