Query to find Duplicate Records in Table? | Query to Delete Duplicate Records from table

  • Interview Question: How to Find Duplicate Records in Table?

Answer: To Find Duplicate Records in table you can use following query:

         select a.* from Employee a where rowid != 
         (select max(rowid) from Employee b where  a.Employee_num =b.Employee_num;

CLICK HERE TO GET 20 COMPLEX SQL QUESTIONS

 

1.Command Used to fetch records:

Select * from Employee;

Employee_num Employee_name Department
1 Rahul OBIEE
1 Rahul OBIEE
2 Rohit OBIEE


  So we will start analysing above table.First we need to calculate the records or
fetch the records which are dupicate records.

 

We are again using concept of row_id here.So i am displaying row_ids of the
employees.

 

select e.*,e.row_id from Employee e;

 

Employee_num Employee_name Department Row_ID
1 Rahul OBIEE 5001
1 Rahul 5002
2 Rohit OBIEE 5003

Here you will see or analyse that for the duplicate records the row_ids are different.So our logic is fetch the records where the row_id is maximum.But we need to take care of joining condition because we want data for specific group.So in our table we will use Employee_num as condition.

So to Fetch the Duplicate records  from table following is the Query:

          select a.* from Employee a where rowid != 
         (select max(rowid) from Employee b where  a.Employee_num =b.Employee_num;

It will fetch following results:

Employee_num Employee_name Department Row_ID
1 Rahul OBIEE 5002

Using Simple delete statement you can remove the duplicate records from the table.

CLICK HERE TO GET 20 IMPORTANT INTERVIEW QUESTIONS FOR TECH MAHINDRA

Use Following Query:

Delete from Employee a where rowid != 

         (select max(rowid) from Employee b where  a.Employee_num =b.Employee_num;

Query 2 : Suggested By Reader Sai Jagdish:

DELETE FROM EMPLOYEE WHERE ROWID IN(SELECT ROWID FROM (SELECT E.*, ROW_NUMBER() OVER (PARTITION BY EMPNO ORDER BY EMPNO) RANK, ROWID FROM EMPLOYEE E) WHERE RANK!=1);

Query 3 : Suggested By Reader Milan Dhore

select Employee_num,count(*) from Employee
group by Employee_num
having count(Employee_num) > 1;

CLICK HERE TO GET INFORMATION ABOUT DIFFERENT ORACLE ERRORS

Hope the query explanation is helpful to you .Please post comment if any questions or queries if you have.These kind of questions are always asked in interviews.

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4 Comments

  1. Sai Jagadish October 30, 2018
    • Amit S October 31, 2018
  2. milan dhore November 9, 2018
    • Amit S November 12, 2018

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