mysql的 if else , case when then， IFNULL

表結構和數據

CREATE TABLE `score`  (
  `s_id` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL,
  `c_id` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL,
  `s_score` int(3) NULL DEFAULT NULL,
  PRIMARY KEY (`s_id`, `c_id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;

INSERT INTO `score` VALUES ('01', '01', 80);
INSERT INTO `score` VALUES ('01', '02', 90);
INSERT INTO `score` VALUES ('01', '03', 99);
INSERT INTO `score` VALUES ('02', '01', 70);
INSERT INTO `score` VALUES ('02', '02', 60);
INSERT INTO `score` VALUES ('02', '03', 80);
INSERT INTO `score` VALUES ('03', '01', 80);
INSERT INTO `score` VALUES ('03', '02', 80);
INSERT INTO `score` VALUES ('03', '03', 80);
INSERT INTO `score` VALUES ('04', '01', 50);
INSERT INTO `score` VALUES ('04', '02', 30);
INSERT INTO `score` VALUES ('04', '03', 20);
INSERT INTO `score` VALUES ('05', '01', 76);
INSERT INTO `score` VALUES ('05', '02', 87);
INSERT INTO `score` VALUES ('06', '01', 31);
INSERT INTO `score` VALUES ('06', '03', 34);
INSERT INTO `score` VALUES ('07', '02', 89);
INSERT INTO `score` VALUES ('07', '03', 98);
INSERT INTO `score` VALUES ('1', '2', 300);
INSERT INTO `score` VALUES ('1', '3', 300);
INSERT INTO `score` VALUES ('1', '4', 300);
INSERT INTO `score` VALUES ('1003', '4', null);
INSERT INTO `score` VALUES ('1004', '3', null);

一、if else

SELECT
	s_id,
	s_score, --這個課可以不用查詢出來下面的if else也能用
IF
	( s_score > 80, '優秀', '良好' ) AS 等級  -- 這個等級就是別名，也能起成s_score
FROM
	score

二、case when then

SELECT 
	s_id,
	s_score,--可以不用寫，下面也能用
		(CASE 
					WHEN s_score>=100 THEN '優秀'
					WHEN s_score>=80 THEN '良好'
					WHEN s_score>=60 THEN '中等'
					ELSE'良好'
			END ) 等級-- 等級只是一個別名而已
 FROM score 

三、 IFNULL

	
SELECT
	s_id,
	IFNULL( s_score, '沒有成績' ) score 
FROM
	score