특수 쿼리 CASE, DECODE

CASE는 자바(JAVA)의 SWITCH문과 비슷하다.

DECODE()는 CASE에서 WHEN THEN END 를 생략한것과 비슷하다.

-- CASE == 
SELECT employee_id, first_name, phone_number,
    CASE SUBSTR(phone_number, 1, 3)
        WHEN '515' THEN '서울'
        WHEN '590' THEN '부산'
        WHEN '650' THEN '광주'
        ELSE '기타'
    END as 지역       
FROM employees;
 
SELECT employee_id, first_name, phone_number,
    CASE
        WHEN SUBSTR(phone_number, 1, 3) = '515' THEN '서울'
        WHEN SUBSTR(phone_number, 1, 3) = '590' THEN '부산'
        WHEN SUBSTR(phone_number, 1, 3) = '650' THEN '광주'
        ELSE '기타'
    END AS 지역
FROM employees;
 
-- DECODE
SELECT employee_id, first_name, phone_number,
    DECODE( SUBSTR(phone_number, 1, 3), 
            '515', '서울', 
            '590', '부산', 
            '650', '광주', 
            '기타') AS 지역
FROM employees;