3단조인

 

JOB_HISTORY 테이블 데이터 

 

 

 

 

EMPLOYEES 테이블의 데이터 

 

 

 

DEPARTMENTS도 가치 표시해줄려고한다 그럼 3단 JOIN을 해야함 

 

 select J.EMPLOYEE_ID, J.START_DATE , J.END_DATE, E.FIRST_NAME, J.DEPARTMENT_ID , D.DEPARTMENT_NAME 

  FROM EMPLOYEES E RIGHT JOIN JOB_HISTORY J ON E.EMPLOYEE_ID = J.EMPLOYEE_ID LEFT JOIN DEPARTMENTS D ON J.DEPARTMENT_ID = D.DEPARTMENT_ID; 

 

자 해석해보면 

EMPYEES 와 JOB_HISTORY중 JOB기준으로 조인을 한 결과값에 DEPARTMENTS조인을 한다. 전체 기준은 JOB_HISTORY 이다. 

 

 

여기서  

DEPARTMENT로 기준으로 바꾸게 되면 

 

 select J.EMPLOYEE_ID, J.START_DATE , J.END_DATE, E.FIRST_NAME, J.DEPARTMENT_ID , D.DEPARTMENT_NAME 

  FROM EMPLOYEES E RIGHT JOIN JOB_HISTORY J ON E.EMPLOYEE_ID = J.EMPLOYEE_ID RIGHT JOIN DEPARTMENTS D ON J.DEPARTMENT_ID = D.DEPARTMENT_ID; 

 

 

총기준은 부서이다.