How to select only the first rows for each unique value of a column?

Let’s say I have a table of customer addresses:

+-----------------------+------------------------+
|         CName         |      AddressLine       |
+-----------------------+------------------------+
|  John Smith           |  123 Nowheresville     |
|  Jane Doe             |  456 Evergreen Terrace |
|  John Smith           |  999 Somewhereelse     |
|  Joe Bloggs           |  1 Second Ave          |
+-----------------------+------------------------+

In the table, one customer like John Smith can have multiple addresses. I need the SELECT query for this table to return only first row found where there are duplicates in ‘CName’. For this table it should return all rows except the 3rd (or 1st – any of those two addresses are okay but only one can be returned).

Is there a keyword I can add to the SELECT query to filter based on whether the server has already seen the column value before?

Answer

A very simple answer if you say you don’t care which address is used.

SELECT
    CName, MIN(AddressLine)
FROM
    MyTable
GROUP BY
    CName

If you want the first according to, say, an “inserted” column then it’s a different query

SELECT
    M.CName, M.AddressLine,
FROM
    (
    SELECT
        CName, MIN(Inserted) AS First
    FROM
        MyTable
    GROUP BY
        CName
    ) foo
    JOIN
    MyTable M ON foo.CName = M.CName AND foo.First = M.Inserted