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removing duplicates


removing duplicates

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Steve Jones
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Show how to find duplicates based on another field. This can be the primary key, but perhaps not. Perhaps it's a timestamp of some sort.

Show how to query to check and then construct a delete statement based on the select.

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You mean use another existing field to differentiate between duplicates in the rest of the data?

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Steve Jones
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Yes, as you could choose a primary key, or if one doesn't exist (or isnt' appropriate) another one. This one is really to teach how someone that doesn't know how to remove a duplicate can do so.

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Does the removing the duplicates job include adding a constraint to make sure the problem doesn't recur?

Tom

Steve Jones
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Worth mentioning, but not necessary. That's not always a possibility.

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L' Eomot Inversé (9/7/2013)
Does the removing the duplicates job include adding a constraint to make sure the problem doesn't recur?


I agree that it probably should at least have an honorable mention... except maybe on staging tables that take in data from 3rd party sources.

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First step towards the paradigm shift of writing Set Based code:
Stop thinking about what you want to do to a row... think, instead, of what you want to do to a column.
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Steve,
It is mentioned at
http://www.sqlservercentral.com/Forums/Topic793765-145-1.aspx
It uses CTE with delete. Very neat.

create table t(a varchar(1))
insert into t values('b')
insert into t values('a')
insert into t values('b')
insert into t values('a')

with
dup as
(
select a,row_number() over(partition by a order by a) rn
from t
)
delete from dup where rn>1


In Oracle this would be pretty straight forward via the use of the pseudo rowid column that all tables have.
This references a physical (and thus unique) address of each row.

At this link
http://www.codeproject.com/Articles/159785/Physical-location-of-a-row-in-SQL-Server
there is an interesting discussion of undocumented %%physloc%% and %%lockres%% columns starting with SS2008R2.
Steve Jones
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Bump

Looking for a writeup here as an article.

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Eirikur Eiriksson
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Would this be of an interest?

Data de-duplication using 2012 window functions.

Simple cases;
1. A set with no surrogate key defined
2. A set with a surrogate key, related records must be updated
Complex cases;
1. Retaining the first value by any chosen sequence
2. Retaining the first good value by any chosen sequence and pattern
3. Retaining the last value by any chosen sequence
4. Retaining the last good value by any chosen sequence and pattern
5. Retaining the most dense value by any chosen pattern
6. Retaining the least dense value by any chosen pattern
Business rule application (possibly a separate article);
1. Rule controlled matching
2. Rule controlled de-duplication
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These would be good. We'd be looking for 2-3 per article to keep this relatively short.

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Steve, here is a step by step article I wrote on how to remove duplicates from a table in SQL Server. Still waiting for approval for the previous submitted articles, so posting the original link for my website for now. see if this helps.

http://sqlsaga.com/sql-server/how-to-remove-duplicates-from-a-table-in-sql-server/


Good Luck Smile .. Visit www.sqlsaga.com for more t-sql code snippets and BI related how to articles.
Steve Jones
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Eirikur Eiriksson (2/16/2014)
Would this be of an interest?

Data de-duplication using 2012 window functions.

Simple cases;
1. A set with no surrogate key defined
2. A set with a surrogate key, related records must be updated
Complex cases;
1. Retaining the first value by any chosen sequence
2. Retaining the first good value by any chosen sequence and pattern
3. Retaining the last value by any chosen sequence
4. Retaining the last good value by any chosen sequence and pattern
5. Retaining the most dense value by any chosen pattern
6. Retaining the least dense value by any chosen pattern
Business rule application (possibly a separate article);
1. Rule controlled matching
2. Rule controlled de-duplication


I think the simple cases are covered.

It would be good to see an article that deletes the first or last value by some sequence. A separate piece could deal with having a pattern.

I hadn't thought about density, but there's an article in there as well.

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