==== 그냥 참조용. ==== SQL> select department_id from employees; DEPARTMENT_ID ------------- 90 90 90 60 60 60 50 50 50 50 50 80 80 80 10 20 20 110 110 20 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 1445457117 ------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 20 | 60 | 3 (0)| 00:00:01 | | 1 | TABLE ACCESS FULL| EMPLOYEES | 20 | 60 | 3 (0)| 00:00:01 | ------------------------------------------------------------------------------- SQL> ed Wrote file afiedt.buf 1 select department_id from employees 2* group by department_id SQL> / DEPARTMENT_ID ------------- 90 20 110 50 80 60 10 8 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 1192169904 -------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 7 | 21 | 4 (25)| 00:00:01 | | 1 | HASH GROUP BY | | 7 | 21 | 4 (25)| 00:00:01 | | 2 | TABLE ACCESS FULL| EMPLOYEES | 20 | 60 | 3 (0)| 00:00:01 | -------------------------------------------------------------------------------- SQL> select distinct department_id from employees; DEPARTMENT_ID ------------- 90 20 110 50 80 60 10 8 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 3042654289 -------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 7 | 21 | 4 (25)| 00:00:01 | | 1 | HASH UNIQUE | | 7 | 21 | 4 (25)| 00:00:01 | | 2 | TABLE ACCESS FULL| EMPLOYEES | 20 | 60 | 3 (0)| 00:00:01 | --------------------------------------------------------------------------------