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;

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1.Command Used to fetch records:

Select * from Employee;

  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;

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:

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;

On Request of Reader :

Query to Find duplicate records in Mysql :

Lets say User wants to find out duplicate Employee Number records from Employee table,

SELECT

    Employee_Num,

    COUNT(Employee_Num)

FROM

    Employees

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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