Oracle: DISTINCT or GROUP BY row consistency - sql

Question

I have following table:
Name Parent Status
A P1 0
A P2 1
B PB -1
Will following queue guarantee, that resulting data will be related to a single row:
SELECT
DISTINCT Name, Parent, Status
FROM
MyTable
For ex. could result set contain:
A, P1, 1
It doesn't match any row in the table. How can write an SQL statement, that selects ANY and AT MOST ONE row with each name?

Answer 1

SQL Fiddle
SELECT DISTINCT will get every row and then discard any duplicate rows in the result set.
The data you've given has duplicates in a column but no duplicate rows - so all rows will be returned:
Query 1:
SELECT DISTINCT
Name,
Parent,
Status
FROM MyTable
Results:
| NAME | PARENT | STATUS |
|------|--------|--------|
| A | P2 | 1 |
| B | PB | -1 |
| A | P1 | 0 |
For ex. could result set contain:
A, P1, 1
No, you can see from the above results that it does not. However, you can make a query that does:
Query 2:
SELECT Name,
MIN( Parent ),
MAX( Status )
FROM MyTable
GROUP BY Name
Results:
| NAME | MIN(PARENT) | MAX(STATUS) |
|------|-------------|-------------|
| A | P1 | 1 |
| B | PB | -1 |
In answer to your final question:
How can write an SQL statement, that selects ANY and AT MOST ONE row with each name?
This query orders the rows randomly and then selects the (randomly) first one for each name:
Query 3:
WITH Randomness AS (
SELECT Name,
Parent,
Status,
ROW_NUMBER() OVER ( PARTITION BY Name ORDER BY SYS.DBMS_RANDOM.VALUE() ) AS Random_ID
FROM MyTable
)
SELECT Name,
Parent,
Status
FROM Randomness
WHERE Random_ID = 1
Results:
| NAME | PARENT | STATUS |
|------|--------|--------|
| A | P1 | 0 |
| B | PB | -1 |
If you run Query 3 a second time then you may get the other A row returned (or not - it's random).
Or if you want to be rather silly and completely random then you can select a random parent and a random status for each name (such that the Parent and Status do not have to come from the same row of the original table).
Query 4:
SELECT Name,
MIN( Parent ) KEEP ( DENSE_RANK FIRST ORDER BY SYS.DBMS_RANDOM.VALUE() ) AS Random_Parent,
MIN( Status ) KEEP ( DENSE_RANK FIRST ORDER BY SYS.DBMS_RANDOM.VALUE() ) AS Random_Status
FROM MyTable
GROUP BY Name
Results:
| NAME | RANDOM_PARENT | RANDOM_STATUS |
|------|---------------|---------------|
| A | P1 | 1 |
| B | PB | -1 |

Answer 2

Please try:
SELECT
Name,
Parent,
Status
FROM(
select
Name,
Parent,
Status,
ROW_NUMBER()
OVER (PARTITION BY Name order by Status desc) RNum
From YourTable
)x where RNum=1
SQL Fiddle Demo