9. CASE

1. CASE 표현식 
    1) 조건 연산자 
    2) IF/ELSE 문과 같은 로직을 구사할수 있음.
    3) 조건문 처리를 할수 있음.
    4) 문법 
        SELECT                                                      <설명>
                CASE WHEN 조건식1 THEN 결과1                    WHEN은 IF와 유사
                     WHEN 조건식2 THEN 결과2                    WHEN은 ELSE IF와 유사
                     ELSE 결과3                                 ELSE는 ELSE와 유사
                END