MySQL中关于case when的用法

CASE [col_name] WHEN [value1] THEN [result1]…ELSE [default] END

CASE WHEN [expr] THEN [result1]…ELSE [default] END

CASE [col_name] WHEN [value1] THEN [result1]…ELSE [default] END​​​​​​​

SELECT
	NAME '英雄',
	CASE NAME
		WHEN '德莱文' THEN
			'斧子'
		WHEN '德玛西亚-盖伦' THEN
			'大宝剑'
		WHEN '暗夜猎手-VN' THEN
			'弩'
		ELSE
			'无'
	END '装备'
FROM
	user_info;

CASE WHEN [expr] THEN [result1]…ELSE [default] END​​​​​​​

# when 表达式中可以使用 and 连接条件
SELECT
	NAME '英雄',
	age '年龄',
	CASE
		WHEN age < 18 THEN
			'少年'
		WHEN age < 30 THEN
			'青年'
		WHEN age >= 30
		AND age < 50 THEN
			'中年'
		ELSE
			'老年'
	END '状态'
FROM
	user_info;

SELECT
	st.stu_id '学号',
	st.stu_name '姓名',
	sum(
		CASE co.course_name
		WHEN '大学语文' THEN
			sc.scores
		ELSE
			0
		END
	) '大学语文',
	sum(
		CASE co.course_name
		WHEN '新视野英语' THEN
			sc.scores
		ELSE
			0
		END
	) '新视野英语',
	sum(
		CASE co.course_name
		WHEN '离散数学' THEN
			sc.scores
		ELSE
			0
		END
	) '离散数学',
	sum(
		CASE co.course_name
		WHEN '概率论与数理统计' THEN
			sc.scores
		ELSE
			0
		END
	) '概率论与数理统计',
	sum(
		CASE co.course_name
		WHEN '线性代数' THEN
			sc.scores
		ELSE
			0
		END
	) '线性代数',
	sum(
		CASE co.course_name
		WHEN '高等数学' THEN
			sc.scores
		ELSE
			0
		END
	) '高等数学'
FROM
	edu_student st
LEFT JOIN edu_score sc ON st.stu_id = sc.stu_id
LEFT JOIN edu_courses co ON co.course_no = sc.course_no
GROUP BY
	st.stu_id
ORDER BY
	NULL;

-- 创建表  学生表
CREATE TABLE `edu_student` (
	`stu_id` VARCHAR (16) NOT NULL COMMENT '学号',
	`stu_name` VARCHAR (20) NOT NULL COMMENT '学生姓名',
	PRIMARY KEY (`stu_id`)
) COMMENT = '学生表' ENGINE = INNODB;

-- 课程表 
CREATE TABLE `edu_courses` (
	`course_no` VARCHAR (20) NOT NULL COMMENT '课程编号',
	`course_name` VARCHAR (100) NOT NULL COMMENT '课程名称',
	PRIMARY KEY (`course_no`)
) COMMENT = '课程表' ENGINE = INNODB;

-- 成绩表
CREATE TABLE `edu_score` (
	`stu_id` VARCHAR (16) NOT NULL COMMENT '学号',
	`course_no` VARCHAR (20) NOT NULL COMMENT '课程编号',
	`scores` FLOAT NULL DEFAULT NULL COMMENT '得分',
	PRIMARY KEY (`stu_id`, `course_no`)
) COMMENT = '成绩表' ENGINE = INNODB;

-- 插入数据

-- 学生表数据

INSERT INTO edu_student (stu_id, stu_name)
VALUES
	('1001', '盲僧'),
	('1002', '赵信'),
	('1003', '皇子'),
	('1004', '寒冰'),
	('1005', '蛮王'),
	('1006', '狐狸');

-- 课程表数据 
INSERT INTO edu_courses (course_no, course_name)
VALUES
	('C001', '大学语文'),
	('C002', '新视野英语'),
	('C003', '离散数学'),
	(
		'C004',
		'概率论与数理统计'
	),
	('C005', '线性代数'),
	('C006', '高等数学');

-- 成绩表数据
INSERT INTO edu_score (stu_id, course_no, scores)
VALUES
	('1001', 'C001', 67),	('1002', 'C001', 68),	('1003', 'C001', 69),	('1004', 'C001', 70),	('1005', 'C001', 71),
	('1006', 'C001', 72),	('1001', 'C002', 87),	('1002', 'C002', 88),	('1003', 'C002', 89),	('1004', 'C002', 90),
	('1005', 'C002', 91),	('1006', 'C002', 92),	('1001', 'C003', 83),	('1002', 'C003', 84),	('1003', 'C003', 85),
	('1004', 'C003', 86),	('1005', 'C003', 87),	('1006', 'C003', 88),	('1001', 'C004', 88),	('1002', 'C004', 89),
	('1003', 'C004', 90),	('1004', 'C004', 91),	('1005', 'C004', 92),	('1006', 'C004', 93),	('1001', 'C005', 77),
	('1002', 'C005', 78),	('1003', 'C005', 79);

CREATE TABLE `tb_hotel_user`  (
  `customer_id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT '旅客id',
  `name` varchar(200) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL COMMENT '姓名',
  `user_id` bigint(10) NULL DEFAULT NULL COMMENT '用户id',
  `check_in_time` datetime(0) NULL DEFAULT NULL COMMENT '入住时间',
  `check_out_time` datetime(0) NULL DEFAULT NULL COMMENT '离店时间',
  PRIMARY KEY (`customer_id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 6 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Compact;

INSERT INTO `tb_hotel_user` VALUES (1, '张三', 1, '2019-12-02 14:18:57', NULL);
INSERT INTO `tb_hotel_user` VALUES (2, '刘大', 2, '2019-11-08 14:19:07', NULL);
INSERT INTO `tb_hotel_user` VALUES (3, '关二', 3, '2019-10-17 14:19:21', NULL);
INSERT INTO `tb_hotel_user` VALUES (4, '关二', 3, '2019-12-02 14:19:44', NULL);
INSERT INTO `tb_hotel_user` VALUES (5, '赵四', 4, '2019-11-29 14:19:54', NULL);


-- 答案
SELECT
	a.user_id,
CASE
	
	WHEN ISNULL( b.user_id ) THEN
	1 ELSE 2 
	END newUser 
FROM
	( SELECT DISTINCT user_id FROM tb_hotel_user WHERE check_in_time >= '2019-10-21 00:00:00' AND check_in_time <= '2019-12-02 23:59:59' ) a
LEFT JOIN ( SELECT user_id FROM tb_hotel_user WHERE check_in_time <= '2019-12-02 23:59:59' GROUP BY user_id HAVING count( * ) > 1 ) b ON a.user_id = b.user_id