Search notes:

Oracle: SELECT DISTINCT

create table tq84_distinct (
       a     number
);

insert into tq84_distinct values (1);
insert into tq84_distinct values (1);

insert into tq84_distinct values (2);

insert into tq84_distinct values (3);

insert into tq84_distinct values (4);
insert into tq84_distinct values (4);

select distinct a from tq84_distinct;

drop table tq84_distinct purge;
Github repository Oracle-Patterns, path: /SQL/select/distinct/distinct_01.sql

Distinct with parantheses

create table tq84_distinct (
       a     number,
       b     number,
       c     number
);

insert into tq84_distinct values (1, 1, 1);
insert into tq84_distinct values (1, 1, 1);
insert into tq84_distinct values (1, 1, 1);

insert into tq84_distinct values (2, 2, 2);
insert into tq84_distinct values (2, 2, 3);

insert into tq84_distinct values (3, 3, 3);
insert into tq84_distinct values (3, 4, 4);

insert into tq84_distinct values (4, 1, 2);
insert into tq84_distinct values (4, 3, 4);
insert into tq84_distinct values (4, 5, 6);

-- Note: the paranthesis around the `a` is
--       misleading. They belong to the `a`,
--       not the the distinct.

select distinct (a),
       b,
       c
  from tq84_distinct
 order by a, b, c;

prompt
prompt

select distinct 
       a,
       b,
       c
  from tq84_distinct
 order by a, b, c;

drop table tq84_distinct purge;
Github repository Oracle-Patterns, path: /SQL/select/distinct/distinct_with_parantheses.sql

SQL Execution plan

The «distinct» of a select distinct query is peformed by the plan operator HASH UNIQUE. 
create table tq84_tab (
   id  number,
   txt varchar2(20),
   val number
);

explain plan for
   select distinct
      txt, val
   from
      tq84_tab
;

select * from table(dbms_xplan.display(format=>'basic'));
--
-- ---------------------------------------
-- | Id  | Operation          | Name     |
-- ---------------------------------------
-- |   0 | SELECT STATEMENT   |          |
-- |   1 |  HASH UNIQUE       |          |
-- |   2 |   TABLE ACCESS FULL| TQ84_TAB |
-- ---------------------------------------

drop table tq84_tab;

See also

ORA-01791: not a SELECTed expression
SELECT statement

Index

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