Question

I have the following data in a simple table:

ID | TYPE
---------
1  | 1
1  | 2
2  | 1
3  | 2
4  | 1
4  | 2
4  | 3
5  | 1
5  | 3
6  | 3

I now need to query this data in the following way:

• For each ID, only one row should be returned
  • If there is only one row for the ID return it.
  • If there are multiple rows for the ID, return the one with TYPE = 2. If that doesn’t exist, return the one with TYPE = 3.
• TYPE can be either 1, 2 or 3
• ID and TYPE have a composite unique index, meaning that per ID there can be a maximum of three rows.

Question: How would I put this logic in a query?

---

Expected result:

ID | TYPE
---------
1  | 2
2  | 1
3  | 2
4  | 2
5  | 3
6  | 3

Answer 1

Oracle has analytic functions in all recent versions.

WITH
  sequenced AS
(
  SELECT
    *,
    ROW_NUMBER() OVER (PARTITION BY id
                           ORDER BY CASE type WHEN 2 THEN 1
                                              WHEN 3 THEN 2
                                              WHEN 1 THEN 3 END) AS sequenced_id
  FROM
    yourTable
)
SELECT
  *
FROM
  sequenced
WHERE
  sequence_id = 1

With this technique you will return all the values in the table, rather than just the PrimaryKey and the search value.

EDIT:

The best way of dealing with that is to have another table that has the order of preference for each type.

 type | preference_value
------+------------------
  2   |         1
  3   |         2
  1   |         3

This states that 2 is the most preferential, 3 is the second preference, and 1 is the third preference.

Because it’s such a small group, however, it can also be done just with a simple CASE statement. See my amended query above.