How to Find Duplicate Values in SQL

Duplicate records waste time, space, and money. Learn how to find and fix duplicate values using SQL’s GROUP BY and HAVING clauses.

The best way to practice GROUP BY and HAVING clauses is LearnSQL.com's interactive SQL Practice Set course. It contains over 80 hands-on exercises to let you practice different SQL constructions, from simple single-table queries, through aggregation, grouping, and joining data from multiple tables, to complex topics such as subqueries.

Database best practices usually dictate having unique constraints (such as the primary key) on a table to prevent the duplication of rows when data is extracted and consolidated. However, you may find yourself working on a dataset with duplicate rows. This could be because of human error, an application bug, or uncleaned data that’s been extracted and merged from external sources, among other things.

Why fix duplicate values? They can mess up calculations. They can even cost a company money; for example, an e-commerce business might process duplicated customer orders multiple times, which can have a direct impact on the business’s bottom line.

In this article, we will discuss how you can find those duplicates in SQL by using the GROUP BY and HAVING clauses.

How to Find Duplicate Values in SQL

First, you will need to define the criteria for detecting duplicate rows. Is it a combination of two or more columns where you want to detect duplicate values, or are you simply searching for duplicates within a single column?

In the examples below, we will be exploring both these scenarios using a simple customer order database.

In terms of the general approach for either scenario, finding duplicates values in SQL comprises two key steps:

1. Using the GROUP BY clause to group all rows by the target column(s) – i.e. the column(s) you want to check for duplicate values on.
2. Using the COUNT function in the HAVING clause to check if any of the groups have more than 1 entry; those would be the duplicate values.

For a quick visual refresher on GROUP BY, check out our We Learn SQL Series’ SQL GROUP BY video. Our SQL Practice Set course offers over 80 hands-on SQL exercises to practice these concepts in great detail.

Duplicate Values in One Column

Here, we will be demonstrating how you can find duplicate values in a single column. For this example, we will be using the Orders table, a modified version of the table we used in my previous article on using GROUP BY in SQL. A sample of the table is shown below.

In this example, there are a few duplicates in the OrderID column. Ideally, each row should have a unique value for OrderID, since each individual order is assigned its own value. For some reason, that wasn’t implemented here. To find the duplicates, we can use the following query:

SELECT OrderID, COUNT(OrderID)
FROM Orders
GROUP BY OrderID
HAVING COUNT(OrderID) > 1

RESULT

Number of Records: 2

As we can see, OrderID 10251 (which we saw in the table sample above) and OrderID 10276 have duplicates.

Using the GROUP BY and HAVING clauses can neatly show the duplicates in your data. Once you have validated that the rows are the same, you may choose to remove the duplicate(s) using the DELETE statement.

Duplicate Values in Multiple Columns

Often, you’re interested in finding rows where a combination of a few columns match. For this example, we will be using the OrderDetails table, a sample of which is shown below.

We want to find entries where the OrderID and ProductID columns are identical. This type of duplicate likely means there is a bug in the ordering system, since each order will process each product in that order only once in the cart. If multiple quantities of that product are ordered, the Quantity value would simply be increased; separate (duplicate) rows should not be created. A glitch of this type may impact business operations negatively if the orders are being fulfilled, packaged, and shipped automatically.

To find duplicates in multiple column values, we can use the following query. It’s very similar to the one for a single column:

SELECT OrderID, ProductID, COUNT(*)
FROM OrderDetails
GROUP BY OrderID, ProductID
HAVING COUNT(*) > 1 

RESULT

Number of Records: 2

Above, we can confirm that the ordering system does indeed have a bug. Like the first example using a single column, this second example similarly allows us to find errors in the ordering system. In this case, products are being registered as a new order even though they were added to the same cart by the same customer. Now you, as the business owner, can take proper corrective actions to rectify this bug in your order management system.

Note that above, we used COUNT(*) and not a column-specific counter such as COUNT(OrderID). COUNT(*) counts all rows, whereas COUNT (Column) only counts non-null values in the specified column. However, in this example, it will not have made a difference either way – there were no null values in either of the two columns being grouped.

Master Dealing with Duplicate Values

Finding duplicates in SQL is mainly about quality/rationality checks and data validation. These checks are often applied to the day-to-day operations of a lot of small and medium businesses.

Also, this is a very common interview question for data science/analyst roles! So, it’s great that you now know the basics of how you can approach this question. Still, it goes without saying that you will definitely need more practice to clearly see the nuances brought forward by the uniqueness of each dataset and what criteria you should be applying for those rationality and quality checks.

To get a better handle on dealing with duplicate records, I would definitely recommend LearnSQL’s SQL Basics course, which covers these concepts holistically, with a comprehensive set of practical exercises.