SQL：Deal With Duplicate Records

• Finding Duplicates with SQL

Here's a handy query for finding duplicates in a table. Suppose you want to find all email addresses in a table that exist more than once:

SELECT email, 
 COUNT(email) AS NumOccurrences
FROM users
GROUP BY email
HAVING ( COUNT(email) > 1 )

You could also use this technique to find rows that occur exactly once:

SELECT email
FROM users
GROUP BY email
HAVING ( COUNT(email) = 1 )

• Delete Duplicate Rows From an Oracle Table

It's easy to introduce duplicate rows of data into Oracle tables by running a data load twice without the primary key or unique indexes created or enabled.
Here's how you remove the duplicate rows before the primary key or unique indexes can be created:

DELETE FROM our_table
WHERE rowid not in
(SELECT MIN(rowid)
FROM our_table
GROUP BY column1, column2, column3... ;

Here column1, column2, column3 constitute the identifying key for each record.
Be sure to replace our_table with the table name from which you want to remove the duplicate rows. The GROUP BY is used on the columns that make the primary key for the table. This script deletes each row in the group after the first row.