I have searched far and wide for an answer to this problem. I'm using a Microsoft SQL Server, suppose I have a table that looks like this:
+--------+---------+-------------+-------------+
| ID | NUMBER | COUNTRY | LANG |
+--------+---------+-------------+-------------+
| 1 | 3968 | UK | English |
| 2 | 3968 | Spain | Spanish |
| 3 | 3968 | USA | English |
| 4 | 1234 | Greece | Greek |
| 5 | 1234 | Italy | Italian |
I want to perform one query which only selects the unique 'NUMBER' column (whether is be the first or last row doesn't bother me). So this would give me:
+--------+---------+-------------+-------------+
| ID | NUMBER | COUNTRY | LANG |
+--------+---------+-------------+-------------+
| 1 | 3968 | UK | English |
| 4 | 1234 | Greece | Greek |
How is this achievable?
This question is related to
sql
sql-server
unique
distinct
Since you don't care, I chose the max ID for each number.
select tbl.* from tbl
inner join (
select max(id) as maxID, number from tbl group by number) maxID
on maxID.maxID = tbl.id
Query Explanation
select
tbl.* -- give me all the data from the base table (tbl)
from
tbl
inner join ( -- only return rows in tbl which match this subquery
select
max(id) as maxID -- MAX (ie distinct) ID per GROUP BY below
from
tbl
group by
NUMBER -- how to group rows for the MAX aggregation
) maxID
on maxID.maxID = tbl.id -- join condition ie only return rows in tbl
-- whose ID is also a MAX ID for a given NUMBER
A very typical approach to this type of problem is to use row_number():
select t.*
from (select t.*,
row_number() over (partition by number order by id) as seqnum
from t
) t
where seqnum = 1;
This is more generalizable than using a comparison to the minimum id. For instance, you can get a random row by using order by newid(). You can select 2 rows by using where seqnum <= 2.
You will use the following query:
SELECT * FROM [table] GROUP BY NUMBER;
Where [table] is the name of the table.
This provides a unique listing for the NUMBER column however the other columns may be meaningless depending on the vendor implementation; which is to say they may not together correspond to a specific row or rows.
Source: Stackoverflow.com