How to find duplicate values in a table

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Vivek johari is currently a Analyst and have more that 5.5 yeras of experience in database. He has Master degree in Computer and also he is Microsoft certified Sql DBA (MCTS)& Microsoft certified SQl BI professional(MCTS). He is also Oracle certified profession(OCP)DBA in ORACLE 10g and ORACLE 9i.He has the experience of working in PL/SQL, T-SQL and SSIS/SSRS. His work basically involved designing and optimization of the Database.He has also published many database articles on his blog Technologies with Vivek Johari.

How to find duplicate values in a table

Posted on 31 January 2010
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Sometimes 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.