What are duplicate records SQL?

Duplication in SQL can also be known as duplicate rows or identic rows. For pairs of identical records, the values in every column will be the same.

How to find duplicates in SQL

It is easy to find duplicates with one field.

Write Query to Verify Duplicates Exist

The 1st query I’m going to write is a simple query to verify whether duplicates exist in our table.

For example

SELECT name, COUNT (email)

From users

Group BY email

HAVING COUNT (email) > 1

So if we have a table as shown is given below

ID        NAME       EMAIL

1           Ali            abc@gmail.com

2          Umar       abc@gmail.com

3           Harry      abc@gmail.com

4           TOM       tom@gmail.com

5            Umar     abc@gmail.com

This will give us Ali, Umar, Harry and Lucky because they all have the same email. As you can see that in ID 2 we have name Umar with email abc@gmail.com and the same thing is happened in ID 5, so from this we can easily find duplicates.

However, if we want to get duplicates with the same email and name, we will get Umar. The reason for getting “Umar” is that I made a mistake, allowed to insert duplicate name and email values.

How to Find Duplicates rows T-SQL?

We need a Select statement to find duplicates rows in a table and that Select statement contains group by with Having keyword. We can also find duplicates with another option and that is to use the ranking function Row_Number(). By using this function we can easily find the duplicates rows in the table. So the above two methods can be used to find duplicates in any table.

Now we will see these two methods one by one.

Find Duplicates rows – Group by

USE model;


Select Name, ID, COUNT(*) CN

FROM Students_Math

GROUP BY name, id


ORDER BY name;


Find Duplicates rows – Row_Number()

USE model;



SELECT name, ID,

Row_Number() OVER(PARTITION BY name, ID ORDER BY name) as CN

FROM Students_Math



How to Find Duplicates in SQL Table

Let’s a schema of a simple table is given below:

Create a Table TableName ( rowid int not null identity (1, 1 ) primary key,

Attr1 varchar ( 20 ) not null,

Attr2 varchar ( 2048 ) not null,

Attr3 tinyint not null

) ;

Now apply this simple and first find duplicates and then delete duplicates from it.

SELECT rowid,
COUNT ( * ) TotalCount
FROM TableName
GROUP BY rowid
HAVING COUNT ( * ) > 1

Above query will find and remove the duplicates from rowid column.

How do I find duplicates in SQL?

How to Find Duplicate Values in SQL
  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.

How do I filter duplicates in SQL?

The go to solution for removing duplicate rows from your result sets is to include the distinct keyword in your select statement. It tells the query engine to remove duplicates to produce a result set in which every row is unique. The group by clause can also be used to remove duplicates.

How do I select only duplicate records in SQL?

How it works:
  1. First, the GROUP BY clause groups the rows into groups by values in both a and b columns.
  2. Second, the COUNT() function returns the number of occurrences of each group (a,b).
  3. Third, the HAVING clause keeps only duplicate groups, which are groups that have more than one occurrence.

How do I find duplicate rows in SQL based on one column?

Find duplicate values in one column
  1. First, use the GROUP BY clause to group all rows by the target column, which is the column that you want to check duplicate.
  2. Then, use the COUNT() function in the HAVING clause to check if any group have more than 1 element. These groups are duplicate.

How do I find duplicate rows in Oracle?

Finding duplicate records using analytic function

In this query, we added an OVER() clause after the COUNT(*) and placed a list of columns, which we checked for duplicate values, after a partition by clause. The partition by clause split rows into groups.

Does Oracle allow duplicate rows?

If the rows are fully duplicated (all values in all columns can have copies) there are no columns to use! But to keep one you still need a unique identifier for each row in each group. Fortunately, Oracle already has something you can use.

How do you eliminate duplicate rows in SQL query without distinct?

Below are alternate solutions :
  1. Remove Duplicates Using Row_Number. WITH CTE (Col1, Col2, Col3, DuplicateCount) AS ( SELECT Col1, Col2, Col3, ROW_NUMBER() OVER(PARTITION BY Col1, Col2, Col3 ORDER BY Col1) AS DuplicateCount FROM MyTable ) SELECT * from CTE Where DuplicateCount = 1.
  2. Remove Duplicates using group By.

What is difference between Rownum and Rowid?

The actual difference between rowid and rownum is, that rowid is a permanent unique identifier for that row. However, the rownum is temporary. If you change your query, the rownum number will refer to another row, the rowid won’t. So the ROWNUM is a consecutive number which applicable for a specific SQL statement only.

Is Rownum stored in a database?

Rowid, Rownum are the Pseudo columns in oracle used to select the data from tables. ROWID is a pseudo column in a table which store and return row address in HEXADECIMAL format with database tables. ROWID is the permanent unique identifiers for each row in the database.

How do I see Rowid in SQL?

ROWID & ROWNUM are pseudocolumns which are not actual columns in the table but behave like actual columns. You can select the values from pseudocolumns like ROWID & ROWNUM. ROWID & ROWNUM are very important pseudocolumns in oracle which is used in data retrieval.

Which is better rank or Dense_rank?

RANK gives you the ranking within your ordered partition. Ties are assigned the same rank, with the next ranking(s) skipped. So, if you have 3 items at rank 2, the next rank listed would be ranked 5. DENSE_RANK again gives you the ranking within your ordered partition, but the ranks are consecutive.

What is difference between rank () Row_number () and Dense_rank () in Oracle?

The row_number gives continuous numbers, while rank and dense_rank give the same rank for duplicates, but the next number in rank is as per continuous order so you will see a jump but in dense_rank doesn’t have any gap in rankings. The row_number() doesn’t break ties and always gives a unique number to each record.

What rank means?

The noun rank refers to a position within a hierarchy, and to rank something is to put it in order — for example, your high school might rank students in terms of their GPAs. You can also use rank to describe an especially foul smell, like the rank gym shoes in the back of your closet.

Why is rank used?

The RANK function is used to retrieve ranked rows based on the condition of the ORDER BY clause. For example, if you want to find the name of the car with third highest power, you can use RANK Function. The PowerRank column in the above table contains the RANK of the cars ordered by descending order of their power.

How do you rank data?

By default, ranks are assigned by ordering the data values in ascending order (smallest to largest), then labeling the smallest value as rank 1. Alternatively, Largest value orders the data in descending order (largest to smallest), and assigns the largest value the rank of 1.

How do you rank rows in SQL?

In the SQL RANK functions, we use the OVER() clause to define a set of rows in the result set. We can also use SQL PARTITION BY clause to define a subset of data in a partition. You can also use Order by clause to sort the results in a descending or ascending order.

How do you calculate rank?