I see a lot of questions/posts about dealing with duplicate records in many SQL Server forums. Many of these questions are asked in SQL Server Interviews and many developers starting their carrier in database programming find it challenging to deal with. Here we will see how we can deal with such records.
Duplicate data is one of the biggest pain points in our IT industry, which various projects have to deal with. Whatever state-of-art technology and best practices followed, the big ERP, CRM, SCM and other inventory based database management projects ends up in having duplicate & redundant data. Duplicate data keeps on increasing by manual entries and automated data loads. Various data leads getting pumped into system’s databases without proper deduping & data cleansing leads to redundant data and thus duplicated record-sets.
Data cleansing requires regular exercise of identifying duplicates, validating and removing them. To minimize these type of scenarios various checks and filters should also be applied before loading new leads into the system.
–> Lets check this by a simple exercise how we can identify & remove duplicate data from a table:
1. Insert some sample records from Person.Contact table of [AdventureWorks] database:
USE [AdventureWorks] GO SELECT TOP 10 ContactID, FirstName, LastName, EmailAddress, Phone INTO DupContacts FROM Person.Contact SELECT * FROM DupContacts
2. Insert some duplicate records from the same list inserted above:
INSERT INTO DupContacts SELECT TOP 50 PERCENT FirstName, LastName, EmailAddress, Phone from DupContacts SELECT * FROM DupContacts
3. Insert some more duplicate records from the same list inserted above.
INSERT INTO DupContacts SELECT TOP 20 PERCENT FirstName, LastName, EmailAddress, Phone from DupContacts SELECT * FROM DupContacts
–> Identify Duplicate records & delete them
Method #1: by using ROW_NUMBER() function:
;WITH dup as ( SELECT ContactID, FirstName, LastName, EmailAddress, Phone, ROW_NUMBER() OVER(PARTITION BY FirstName, LastName ORDER BY ContactID) AS NumOfDups FROM DupContacts) SELECT * FROM dup WHERE NumOfDups > 1 ORDER BY ContactID
-- Remove/Delete duplicate records: ;WITH dup as ( SELECT ContactID, FirstName, LastName, EmailAddress, Phone, ROW_NUMBER() OVER(PARTITION BY FirstName, LastName ORDER BY ContactID) AS NumOfDups FROM DupContacts) DELETE FROM dup WHERE NumOfDups > 1 SELECT * FROM DupContacts
Method #2: by using SELF-JOIN:
SELECT DISTINCT a.ContactID, a.FirstName, a.LastName, a.EmailAddress, a.Phone FROM DupContacts a JOIN DupContacts b ON a.FirstName = b.FirstName AND a.LastName = b.LastName AND a.ContactID > b.ContactID
-- Remove/Delete duplicate records: DELETE a FROM DupContacts a JOIN DupContacts b ON a.FirstName = b.FirstName AND a.LastName = b.LastName AND a.ContactID > b.ContactID SELECT * FROM DupContacts
Method #3: by using AGGREGATES & Sub-QUERY:
SELECT * FROM DupContacts WHERE ContactID NOT IN (SELECT MIN(ContactID) FROM DupContacts GROUP BY FirstName, LastName)
-- Remove/Delete duplicate records: DELETE FROM DupContacts WHERE ContactID NOT IN (SELECT MIN(ContactID) FROM DupContacts GROUP BY FirstName, LastName) SELECT * FROM DupContacts
–> Final Cleanup
DROP TABLE DupContacts
Check the same demo here:
Just replied an answer on a SQL forum (http://www.sqlteam.com/forums/topic.asp?TOPIC_ID=148860), so thought to post this as well for future reference.
How will you output the following record set: 123456788 City 399.99 123456788 County 499.99 123456788 Flood 299.99 123456788 Hazzard 199.99 123456789 City 333.99 123456789 County 444.99 123456789 Flood 222.99 123456789 Hazzard 111.99 ... into following pattern: id Hazzard Flood City County 123456788 199.99 299.99 399.99 499.99 123456789 111.99 222.99 333.99 444.99
The following query will do it and convert the rows into columns:
select 123456788 as id, 'Hazzard' as des, 199.99 as val into #tempTable union select 123456788, 'Flood', 299.99 union select 123456788, 'City', 399.99 union select 123456788, 'County', 499.99 union select 123456789, 'Hazzard', 111.99 union select 123456789, 'Flood', 222.99 union select 123456789, 'City', 333.99 union select 123456789, 'County', 444.99 select * from #tempTable select a.id as id, a.val as Hazzard, b.val as Flood, c.val as City, d.val as County from #tempTable a,#tempTable b,#tempTable c, #tempTable d where a.id=b.id and b.id=c.id and c.id=d.id and a.des='Hazzard' and b.des='Flood' and c.des='City' and d.des='County' drop table #tempTable
The above select code is with a simple old style where clause with multiple tables joining. The code could be also converted to self-joins.