Select distinct('more than one column') from ....

Question

Why does this sql statement not work:
select distinct(column_a, column_b, column_c), empno
from emp;
How can I 'select distinct' using more than one column.

Answer 1

Actually, DISTINCT applies to all columns in the result.
Distinct is not a function, you cannot do distinct(column1, column2, ...).
As a matter of fact it doesn't make sense to expect distinct applies to only one column, because you do not specify any criteria on how to select the values for the second column.
A final note: you can do distinct(column1), column2, but this will still apply to both columns!
Here're some examples that will all return both rows from the set, where set is: (1,1), (1,2):
select distinct val1, val2
from
select 1 as val1, 1 as val2 from dual
union all
select 1 as val1, 2 as val2 from dual
select distinct val1||'-'||val2
from
select 1 as val1, 1 as val2 from dual
union all
select 1 as val1, 2 as val2 from dual
select distinct(val1), val2 -- this is very misleading. distinct still applies to both columns
from
select 1 as val1, 1 as val2 from dual
union all
select 1 as val1, 2 as val2 from dual
Edited by: user4010726 on Dec 1, 2009 11:06 AM
Edited by: user4010726 on Dec 1, 2009 11:07 AM