Question

How do I perform a DISTINCT operation on a single column after a UNION is performed?

T1
--
ID Value 
1  1
2  2
3  3

T2
--
ID Value
1  2
4  4
5  5

I am trying to return the table:

ID Value
1  1
2  2
3  3
4  4
5  5

I tried:

SELECT DISTINCT ID, Value 
FROM (SELECT*FROM T1 UNION SELECT*FROM T2) AS T3

This does not seem to work.

Answer 1

As far as I can say, there’s no “one-column distinct“: distinct is always applied to a whole record (unless used within an aggregate like count(distinct name)). The reason for this is, SQL cannot guess which values of Value to leave for you—and which to drop. That’s something you need to define by yourself.

Try using GROUP BY to ensure ID is not repeated, and any aggregate (here MIN, as in your example it was the minimum that survived) to select a particular value of Value:

SELECT ID, min(Value) FROM (SELECT * FROM T1 UNION ALL SELECT * FROM T2) AS T3
GROUP BY ID

Should be exactly what you need. That is, it’s not the same query, and there’s no distinct—but it’s a query which would return what’s shown in the example.