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부모 자식 관계

독립형 오퍼레이션

SELECT deptno, count(*)
FROM emp WHERE job = 'CLERK'
AND sal < 1200 GROUP BY deptno

-------------------------------------------------------------------
| Id  | Operation                    | Name      | Starts | A-Rows |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT             |           |      1 |      2 |
|   1 |  HASH GROUP BY               |           |      1 |      2 |
|*  2 |   TABLE ACCESS BY INDEX ROWID| EMP       |      1 |      3 |
|*  3 |    INDEX RANGE SCAN          | EMP_JOB_I |      1 |      4 |
-------------------------------------------------------------------

   2 - filter("SAL"<1200)   3 - access("JOB"='CLERK')