MySQL case when 用法

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;
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;

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;
聚合函数 sum 配合 case when 的简单函数实现行转列
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);