Oracle SQL - return record only if colB is the same for all of colA - sql

Question

I have a table like the following ( there is of course other data in the table):
Col A Col B
1 Red
1 Red
2 Blue
2 Green
3 Black
I am trying to return a value for Col A only when ALL the Col B values match, otherwise return null.
This will be used as part of another sql statement that will be passing the Col A value, ie
Select * from Table where Col A = 1
I need to return the value in Col B. The correct result in the above table would be Red,Black
any ideas ?

Answer 1

how about this?
SQL Fiddle
Oracle 11g R2 Schema Setup:
create table t( id number, color varchar2(20));
insert into t values(1,'RED');
insert into t values(1,'RED');
insert into t values(2,'BLUE');
insert into t values(2,'GREEN');
insert into t values(3,'BLACK');
Query 1:
select color from t where id in (
select id
from t
group by id having min(color) = max(color) )
group by color
Results:
| COLOR |
|-------|
| RED |
| BLACK |

Answer 2

If you just want the values in A (rather than each row), then use group by:
select a
from table t
group by a
having min(b) = max(b);
Note: this ignores NULL values. If you want to treat them as an additional value, then add another condition:
select a
from table t
group by a
having min(b) = max(b) and count(*) = count(b);
It is also tempting to use count(distinct). In general, though, count(distinct) requires more processing effort than a min() and a max().

Answer 3

You can use a case statement.
select cola,
case when max(colb) = min(colb) and count(*) = count(colb) then max(colb)
end as colb
from tablename
group by cola

Answer 4

SQL Fiddle
Oracle 11g R2 Schema Setup:
create table t( id number, color varchar2(20));
insert into t values(1,'RED');
insert into t values(1,'RED');
insert into t values(2,'BLUE');
insert into t values(2,'GREEN');
insert into t values(3,'BLACK');
Query 1:
select id
from t
group by id having min(color) = max(color)
Results:
| ID |
|----|
| 1 |
| 3 |
hope this is what you were looking for.. :)