Question

In SQL Server, I have a table where a column A stores some data. This data can contain duplicates (ie. two or more rows will have the same value for the column A).

I can easily find the duplicates by doing:

select A, count(A) as CountDuplicates
from TableName
group by A having (count(A) > 1)

Now, I want to retrieve the values of other columns, let’s say B and C. Of course, those B and C values can be different even for the rows sharing the same A value, but it doesn’t matter for me. I just want any B value and any C one, the first, the last or the random one.

If I had a small table and one or two columns to retrieve, I would do something like:

select A, count(A) as CountDuplicates, (
    select top 1 child.B from TableName as child where child.A = base.A) as B
)
from TableName as base group by A having (count(A) > 1)

The problem is that I have much more rows to get, and the table is quite big, so having several children selects will have a high performance cost.

So, is there a less ugly pure SQL solution to do this?

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Not sure if my question is clear enough, so I give an example based on AdventureWorks database. Let’s say I want to list available States, and for each State, get its code, a city (any city) and an address (any address). The easiest, and the most inefficient way to do it would be:

var q = from c in data.StateProvinces select new { c.StateProvinceCode, c.Addresses.First().City, c.Addresses.First().AddressLine1 };

in LINQ-to-SQL and will do two selects for each of 181 States, so 363 selects. I my case, I am searching for a way to have a maximum of 182 selects.

Answer 1

you can do some thing like this if you have id as primary key in your table

select id,b,c from tablename 
inner join
(
select id, count(A) as CountDuplicates
from TableName as base group by A,id having (count(A) > 1) 
)d on tablename.id= d.id