Question

I have a table defined as this:

CREATE TABLE A_TEST  (
    "ID"   NUMBER NOT NULL ENABLE,
    "TYPE" VARCHAR2(20 BYTE),
)

• ID is not a primary key.
• TYPE might be NULL
• An ID I can have 0, 1 or more TYPEs.

if I want to get IDs with differents types I will write this query:

SELECT distinct a1.ID FROM A_TEST a1
  JOIN A_TEST a2 on a2.ID = a1.ID and a2.type != a1.type

But how can I find Ids for which I have a single type defined?
(only one row for Id or many rows for id with only one having a type defined)

Answer 1

After the clarifications, it seems that your requirements are:

SELECT ID
FROM A_TEST
GROUP BY ID    
HAVING COUNT(DISTINCT TYPE) = 1
    OR COUNT(*) = 1 ;