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.
SELECT name, COUNT (email)
Group BY email
HAVING COUNT (email) > 1
So if we have a table as shown is given below
ID NAME EMAIL
1 Ali email@example.com
2 Umar firstname.lastname@example.org
3 Harry email@example.com
4 TOM firstname.lastname@example.org
5 Umar email@example.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 firstname.lastname@example.org 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
Select Name, ID, COUNT(*) CN
GROUP BY name, id
HAVING COUNT(*) > 1
ORDER BY name;
Find Duplicates rows – Row_Number()
SELECT * FROM (
SELECT name, ID,
Row_Number() OVER(PARTITION BY name, ID ORDER BY name) as CN
) AS Q WHERE Q.CN > 1
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.
COUNT ( * ) TotalCount
GROUP BY rowid
HAVING COUNT ( * ) > 1
ORDER BY COUNT ( * ) DESC
Above query will find and remove the duplicates from rowid column.
How do I find duplicates in SQL?
- 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.
- 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?
How do I select only duplicate records in SQL?
- First, the GROUP BY clause groups the rows into groups by values in both a and b columns.
- Second, the COUNT() function returns the number of occurrences of each group (a,b).
- 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?
- First, use the GROUP BY clause to group all rows by the target column, which is the column that you want to check duplicate.
- 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?
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?
How do you eliminate duplicate rows in SQL query without distinct?
- 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.
- Remove Duplicates using group By.