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

You could use this approach:

SELECT DISTINCT id, attribute 
FROM test t1 
WHERE (SELECT count(DISTINCT attribute) 
       FROM test t2 
       WHERE t2.id = t1.id) = 1

A better approach could be:

SELECT
   DISTINCT t1.id, t1.attribute
FROM
   test t1,
   (
      SELECT
         id,
         count(DISTINCT attribute) COUNT
      FROM
         test
      GROUP BY
         id
      HAVING
         COUNT = 1
   ) t2
WHERE
   t1.id = t2.id