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Tip of the Day
Language: SQL Server
Expertise: Beginner
Aug 8, 2000

Counting Duplicate Rows

Question:
How do I count the number of duplicate items in a table?

Answer:
Lets break down your question into several steps. First, let's create a sample table using the following code:

create table dups
(
	i int
)
go

declare @i int

select @i = 0

while (@i < 35)
begin
   insert into dups(i) values (cast (rand() * 50  as int))
	select  @i = @i + 1
end
Now, let's find rows that are duplicates. For that we can use a simple group by statement:
select i,count(*) as num_records
from dups
group by i
having count(*)  > 1
My sample data produced the following result set:
i           num_records 
----------- ----------- 
0           2
5           2
18          2
22          2
27          2
31          2
34          2
44          2
49          2
This identifies the rows that have duplicates. But it does not return the total number of duplicates in the table. The first change we must make is to recognize the above rows that show 2 contain only one duplicate.

So we want a query that basically sums up the duplicates from the above query. To do so, we take the previous query and can put that in the from statement as a derived table. We then can use the sum function to create the total for us:

select sum(num_dups)
from (select i,count(*)-1 as num_dups
from dups
group by i
having count(*)-1  > 0) as mydups
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