Removing Duplicate Records

December 10, 2000

Introduction

"How do I remove duplicate rows from a table" is a common question in the newsgroups, and here we will look at a few different techniques for achieving this.

Databases can easily be designed to disallow duplicates (which I will discuss at the bottom of the article) but the issue can still arise. For example, duplicate data might arrive at your database via an interface to another system.

The code examples in this article were all developed on SQL 2000, but should work fine in versions 6.5 and 7.

The "Distinct" technique

This is the easiest way of de-duplicating a table, and is at its best with fairly small rowsets, and where all the columns in a table are duplicates. We simply run a SELECT DISTINCT on the offending table, storing the results in a temporary table, then delete the contents of the permanent table and re-populate it from the temporary one.

The downside of this is that you need enough free data space in your database (or in tempdb) to store the entire table again, plus plenty of log space when you are de-duplicating large tables. This simple example loads a table with duplicate data then removes the duplicates:

Click here for code example 1.

Using derived tables

For tables where duplication is defined by a subset of the columns in the table, you can use one of the other columns in the table to identify which rows to keep and which to delete. Here is a simple example--it is similar to the previous one, but note that only the names are duplicated, because the "ident" column has been defined as an Identity column. Here we only want to keep one instance of each name, regardless of the value in the Ident column. In the example I have arbitrarily decided to keep the name with the lowest Ident value

Click here for code example 2.

The inner query selects all the names in the table that are duplicated, and the minimum value of the Ident column for each name. These results are compared against the main table in order to select the records for deletion

With this technique you may be able to get away with having much less free room than the earlier technique, but this depends on the ratio of duplicated to unique records--the more records are duplicated, the more space SQL Server needs to temporarily store, and work with, the results that makeup the derived table. The derived table technique is the one I usually try first.

Using correlated subqueries

This example uses a correlated sub-query to achieve de-duplication

create table marxBrothers (
    ident int IDENTITY,
    Name varchar(32)
)
go
insert marxBrothers (Name)
    select 'Groucho Marx' UNION ALL
    select 'Harpo Marx' UNION ALL
    select 'Chico Marx' UNION ALL
    select 'Groucho Marx' UNION ALL
    select 'Harpo Marx' UNION ALL
    select 'Chico Marx' UNION ALL
    select 'Groucho Marx' UNION ALL
    select 'Harpo Marx' UNION ALL
    select 'Chico Marx' UNION ALL
    select 'Groucho Marx' UNION ALL
    select 'Harpo Marx' UNION ALL
    select 'Chico Marx' UNION ALL
    select 'Zeppo Marx' UNION ALL
    select 'Gummo Marx' UNION ALL
    select 'Zeppo Marx'
  
select * from marxBrothers

delete marxBrothers
where ident > (
    select min(ident)
    from marxBrothers m
    where m.name = marxBrothers.name
)

select * from marxBrothers

drop table marxBrothers

Correlated sub-queries can be pretty slow and inefficient to run against large tables.

De-duplicating using dynamic SQL

This technique uses a cursor and the "SET ROWCOUNT" feature to work through the table one name at a time, deleting all but one of the rows for each name. Cursors and Dynamic SQL both have their place, but I generally try to avoid using them for reasons of efficiency, As this example uses both a cursor and dynamic SQL, it can bet pretty inefficient with large numbers of duplicated rows, and I would prefer to use one of the previous techniques. However, the technique does work, so I think it is still worth a mention.

create table marxBrothers (
ident int IDENTITY,
Name varchar(32)
)
insert marxBrothers (Name)
    select 'Groucho Marx' UNION ALL
    select 'Groucho Marx' UNION ALL
    select 'Groucho Marx' UNION ALL
    select 'Groucho Marx' UNION ALL
    select 'Harpo Marx' UNION ALL
    select 'Chico Marx' UNION ALL
    select 'Chico Marx' UNION ALL
    select 'Chico Marx' UNION ALL
    select 'Groucho Marx' UNION ALL
    select 'Harpo Marx' UNION ALL
    select 'Chico Marx' UNION ALL
    select 'Zeppo Marx' UNION ALL
    select 'Gummo Marx' UNION ALL
    select 'Zeppo Marx' UNION ALL
    select 'Zeppo Marx'
go

declare @sql varchar(255),
        @name varchar(32),
        @count int
    
select * from marxBrothers

declare CI insensitive cursor for
    select name, count(1) -- 1 from marxBrothers m
    group by name
    having count(1) > 1

open CI
fetch next from CI into @name, @count

while @@fetch_status = 0
begin
    select @sql =
    'set rowcount ' + convert(varchar(9), @count) + '
    delete marxBrothers where name = "' + @name + '"'

    exec (@sql)

fetch next from CI into @name, @count

end-- while fetch

close CI
deallocate CI

select * from marxBrothers

go
drop table marxBrothers

Preventing duplication

Prevention is, after all, better than cure, so It would be pointless discussing de-duplicating a table without mentioning how to prevent duplicate rows appearing again afterwards. There are three main declarative methods for doing this in SQL:

The Primary Key constraint

This is implemented as an index in SQL Server, and so it can also help speed up select and join operations from the table. One or more columns may make up a primary key, and SQL Server will automatically ensure that duplicates do not exist in a table's primary key. An error will be returned if you try to insert duplicates.

create table marxBrothers (
    ident      int IDENTITY,
    name       varchar(32),
    constraint PK_marxBrothers 
PRIMARY KEY (name)
)
go

The Unique constraint

This is also Implemented as an index in SQL Server. The main difference between a primary key and a unique constraint is that the Primary Key can never contain a NULL value.

create table marxBrothers (
    ident         int	IDENTITY,
    name          varchar(32),
    constraint    U_marxBrothers UNIQUE (name)
)
go

The IGNORE_DUP_KEY option

This is an option on the create index command, which will cause SQL Server to discard rows where a duplicate of the columns in the specified index already exists. Use this option with care--you have to be certain that you do really need to do this. Also remember that the results of using this option can easily be misinterpreted as an error by end users.

create table marxBrothers (
    ident         int	IDENTITY,
    name          varchar(32),
)
go
create unique index UI_marxBrothers on marxBrothers (name) with ignore_dup_key