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How to find duplicate values in a table.

, 16 Nov 2010
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This article will try to show, how to find duplicate values in a table.

Duplicate values in tables can create a major problem when we do not make a primary key or a unique key on a table. In these cases we need to find out the duplicate records and need to delete them. We can use the Having Clause to find out that duplicate records.we show this with the help of an example.

Suppose we have a table named EmployeeDeptInfo which have the column Employeid and Departmentid. The query for creating this table is given below:

Create table EmployeeDeptInfo ( Employeeid int, Departmentid int)

Employeeid contains the Id of the employee and Departmentid contains the Id of the department to which he/she is belonged. This table do not have any primary key defined on it and also it doesnt have any unique key constraint defined on any column.

Suppose the table contains the following data

Employeeid Departmentid
1 1
2 2
3 2
4 3
3 2
2 2
5 4
2 2

In this table, entries for the employee having employeeid 2 & 3 are get repeated.In this example the data in the table is not much for the example point of view but in real time scenario it can be billions of rows and duplication of rows can be a very big concern. Therefore it is necessary to find out the duplicate rows in the table. We can use the Having Clause to find out the duplicate rows.

Query for finding out the duplicate rows in the table:

Select Employeeid, Departmentid from EmployeeDeptInfo Group By Employeeid, 
    DepartmentId having (count(employeeid) >1 and count (Departmentid) >1)

This query will give us the following results:

Employeeid Departmentid
2 2
3 2

We can also find out that how many times a record is repeated in the table. For example, The following query gives us the result of how many times a given record is repeated

Select Employeeid, Departmentid, 
    count (Employeeid) as NoOfRepeat from EmployeeDeptInfo Group By Employeeid, 
    DepartmentId having (count(employeeid) >1 and count (Departmentid) >1)

This query will give us the following results

Employeeid Departmentid NoOfRepeat
2 2 3
3 2 2

Here NoOfRepeat shows the number of times the records are duplicated in the table

Summary

This article shows that we can find the duplicate records with the help of the Group By and Having Clause.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Vivek Johari
Technical Lead
India India
I am currently working as a Analyst and have around 7.5 years of experience in database.
 
Degree:-
Master Degree in Computer(MCA)
 
Work experience:-
Designing of the database.
Database Optimization.
Writing Complex Stored Procedures,Functions,Triggers etc.
Designing and developing SSIS & DTS packages.
Designing SQL Reports using SSRS.
Database Server Maintenance.
 
Certification:-
Microsoft certified Sql DBA in Sql server 2008 (MCTS).
Microsoft certified BI professional in Sql server 2008 (MCTS).
Oracle certified profession DBA in ORACLE 10g (OCP)
certified profession DBA in ORACLE 9i (OCP)
 
My other publication
Technical Blog:- Technologies with Vivek Johari
 
Moderator and Blogger at BeyondRelational.com
 
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Comments and Discussions

 
GeneralMy vote of 5 PinmemberOshtri Deka5-May-11 1:42 
GeneralRe: My vote of 5 PinmemberVivek Johari3-Nov-11 1:29 
GeneralMy vote of 3 PinmemberBloodyBaron30-Jan-11 8:25 
GeneralRe: My vote of 3 PinmemberVivek Johari3-Nov-11 1:29 
GeneralMy vote of 3 PinmemberDilip Baboo22-Nov-10 6:32 
GeneralRe: My vote of 3 PinmemberFlamewave422-Nov-10 8:43 
GeneralExcellent Tip PinmemberFlamewave422-Nov-10 5:22 
GeneralRe: Excellent Tip PinmemberVivek Johari22-Nov-10 8:04 

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