Check column for unique value

Question

My table:

id attribute
1  2
1  2
2  3
2  4
5  1
5  1
6  3
6  3
6  5

Now I want only to output those id with attribute, if the attribute is the same for each id.

In this sample table, the output would be

id attribute
1  2
5  1

Answer 1

SELECT id, MIN(attribute) AS attribute 
FROM test  
GROUP BY id
HAVING COUNT(DISTINCT attribute) = 1 ;

or:

SELECT id, MIN(attribute) AS attribute
FROM test 
GROUP BY id
HAVING MIN(attribute) = MAX(attribute) ;

I would expect the last version to be quite efficient with an index on (id, attribute)