5 ways to delete duplicate records Oracle

In Oracle there are many ways to delete duplicate records. Note that below example are described to just explain the different possibilities.

Consider the EMP table with below rows

create table emp(
EMPNNO  integer,
EMPNAME varchar2(20),
SALARY  number);

10    Bill    2000
11    Bill    2000
12    Mark    3000
12    Mark    3000
12    Mark    3000
13    Tom    4000
14    Tom    5000
15    Susan    5000

1. Using rowid

SQL > delete from emp
where rowid not in
(select max(rowid) from emp group by empno);

This technique can be applied to almost scenarios. Group by operation should be on the columns which identify the duplicates.

2. Using self-join

SQL > delete from emp e1
where rowid not in
(select max(rowid) from emp e2
where e1.empno = e2.empno );

3. Using row_number()

SQL > delete from emp where rowid in
(
select rid from
(
select rowid rid,
row_number() over(partition by empno order by empno) rn
from emp
)
where rn > 1
);

This is another efficient way to delete duplicates

4. Using dense_rank()

SQL > delete from emp where rowid in
(
select rid from
(
select rowid rid,
dense_rank() over(partition by empno order by rowid) rn
from emp
)
where rn > 1
);

Here you can use both rank() and dens_rank() since both will give unique records when order by rowid.

5. Using group by

Consider the EMP table with below rows

10    Bill    2000
11    Bill    2000
12    Mark    3000
13    Mark    3000

SQL > delete from emp where
(empno,empname,salary) in
(
select max(empno),empname,salary from emp
group by empname,salary
);

This technique is only applicable in few scenarios.

Always take extra caution while deleting records. 

1. First identify the duplicates using select.

2. Double verify those are actual  ‘duplicates’ or not

3. Take backup if necessary

4. Apply commit only if you are sure.

Did you find above post useful ? Your comments are highly valuable.

What is difference between View and Materialized View in Database or SQL?

Difference between View and Materialized view is one of the popular SQL interview question, much like truncate vs delete,  correlated vs noncorrelated subquery or primary key vs unique key.  This is one of the classic question which keeps appearing in SQL interview now and then and you simply can’t afford not to learn about them. Doesn’t matter if you are a programmer, developer or DBA, this SQL questions is common to all. Views are concept which not every programmer familiar of, it simply not in the category of CRUD operation or database transactions or SELECT query, its little advanced concept for average programmer. Views allows a level of separation than original table in terms of access rights but it always fetch updated data. Let’s see What is View in database, What is materialized View and difference between view and materialized view in database.

What is View in database

Views are logical virtual table created by “select query” but the result is not stored anywhere in the disk and every time we need to fire the query when we need data, so always we get updated or latest data from original tables. Performance of the view depend upon our select query. If we want to improve the performance of view we should avoid to use join statement in our query or if we need multiple joins between table always try to use index based column for joining as we know index based columns are faster than non index based column. View allow to store definition of the query in the database itself.

What is Materialized View in database

Materialized views are also logical view of our data driven by select query but the result of the query will get stored in the table or disk, also definition of the query will also store in the database .When we see the performance of Materialized view it is better than normal View because the data of materialized view will stored in table and table may be indexed so faster for joining also joining is done at the time of materialized views refresh time so no need to every time fire join statement as in case of view.

Difference between View vs Materialized View in database

Based upon on our understanding of View and Materialized View, Let’s see, some short difference between them :

1) First difference between View and materialized view is that, In Views query result is not stored in the disk or database but Materialized view allow to store query result in disk or table.

2) Another difference between View vs materialized view is that, when we create view using any table,  rowid of view is same as original table but in case of Materialized view rowid is different.

3) One more difference between View and materialized view in database is that, In case of View we always get latest data but in case of Materialized view we need to refresh the view for getting latest data.

4) Performance of View is less than Materialized view.

5) This is continuation of first difference between View and Materialized View, In case of view its only the logical view of table no separate copy of table but in case of Materialized view we get physically separate copy of table

6) Last difference between View vs Materialized View is that, In case of Materialized view we need extra trigger or some automatic method so that we can keep MV refreshed, this is not required for views in database.

When to Use View vs Materialized View in SQL

Mostly in application we use views because they are more feasible,  only logical representation of table data no extra space needed. We easily get replica of data and we can perform our operation on that data without affecting actual table data but when we see performance which is crucial for large application they use materialized view where Query Response time matters so Materialized views are used mostly with data ware housing or business intelligence application.