原文链接：https://blog.csdn.net/rongtaoup/article/details/82183743
前言
建议先看看官方文档_对 case when 的用法
CASE SCORE WHEN 'A' THEN '优' ELSE '不及格' END
# 使用 IF 函数进行替换
IF(SCORE = 'A', '优', '不及格')
THEN后边的值与ELSE后边的值类型应一致，否则会报错。
如下：
CASE SCORE WHEN ‘A’ THEN ‘优’ ELSE 0 END’优’和0数据类型不一致则报错：
[Err] ORA-00932: 数据类型不一致: 应为 CHAR, 但却获得 NUMBER
简单CASE WHEN函数只能应对一些简单的业务场景，而CASE WHEN条件表达式的写法则更加灵活。
类似JAVA中的IF ELSE语句。
格式：
CASE WHEN condition THEN result
[WHEN...THEN...]
ELSE result
END
SQL语言演示：
CASE
WHEN SCORE = 'A' THEN '优'
WHEN SCORE = 'B' THEN '良'
WHEN SCORE = 'C' THEN '中'
ELSE '不及格' END
# 等同于
CASE score
WHEN 'A' THEN '优'
WHEN 'B' THEN '良'
WHEN 'C' THEN '中'
ELSE '不及格' END
condition是一个返回布尔类型的表达式，
如果表达式返回true，则整个函数返回相应result的值，
如果表达式皆为false，则返回ElSE后result的值，如果省略了ELSE子句，则返回NULL。
前言
students表的DDL
-- auto-generated definition
create table students
(
stu_code varchar(10) null,
stu_name varchar(10) null,
stu_sex int null,
stu_score int null
);
students表的DML
# 其中stu_sex字段，0表示男生，1表示女生。
INSERT INTO students (stu_code, stu_name, stu_sex, stu_score) VALUES ('xm', '小明', 0, 88);
INSERT INTO students (stu_code, stu_name, stu_sex, stu_score) VALUES ('xl', '夏磊', 0, 55);
INSERT INTO students (stu_code, stu_name, stu_sex, stu_score) VALUES ('xf', '晓峰', 0, 45);
INSERT INTO students (stu_code, stu_name, stu_sex, stu_score) VALUES ('xh', '小红', 1, 89);
INSERT INTO students (stu_code, stu_name, stu_sex, stu_score) VALUES ('xn', '小妮', 1, 77);
INSERT INTO students (stu_code, stu_name, stu_sex, stu_score) VALUES ('xy', '小一', 1, 99);
INSERT INTO students (stu_code, stu_name, stu_sex, stu_score) VALUES ('xs', '小时', 1, 45);
energy_test表的DDL
-- auto-generated definition
create table energy_test
(
e_code varchar(2) null,
e_value decimal(5, 2) null,
e_type int null
);
energy_test表的DML
# 其中，E_TYPE表示能耗类型，0表示水耗，1表示电耗，2表示热耗
INSERT INTO energy_test (e_code, e_value, e_type) VALUES ('北京', 28.50, 0);
INSERT INTO energy_test (e_code, e_value, e_type) VALUES ('北京', 23.50, 1);
INSERT INTO energy_test (e_code, e_value, e_type) VALUES ('北京', 28.12, 2);
INSERT INTO energy_test (e_code, e_value, e_type) VALUES ('北京', 12.30, 0);
INSERT INTO energy_test (e_code, e_value, e_type) VALUES ('北京', 15.46, 1);
INSERT INTO energy_test (e_code, e_value, e_type) VALUES ('上海', 18.88, 0);
INSERT INTO energy_test (e_code, e_value, e_type) VALUES ('上海', 16.66, 1);
INSERT INTO energy_test (e_code, e_value, e_type) VALUES ('上海', 19.99, 0);
INSERT INTO energy_test (e_code, e_value, e_type) VALUES ('上海', 10.05, 0);
p_price表的DDL
-- auto-generated definition
create table p_price
(
p_price decimal(5, 2) null comment '价格',
p_level int null comment '等级',
p_limit int null comment '阈值'
)
comment '电能耗单价表';
p_price表的DML
INSERT INTO test.p_price (p_price, p_level, p_limit) VALUES (1.20, 0, 10);
INSERT INTO test.p_price (p_price, p_level, p_limit) VALUES (1.70, 1, 30);
INSERT INTO test.p_price (p_price, p_level, p_limit) VALUES (2.50, 2, 50);
user_col_comments 表的DDL
-- auto-generated definition
create table user_col_comments
(
column_name varchar(50) null comment '列名',
comment varchar(100) null comment '列的备注'
);
user_col_comments 表的DML
INSERT INTO test.user_col_comments (column_name, comment) VALUES ('SHI_SHI_CODE', '设施编号');
INSERT INTO test.user_col_comments (column_name, comment) VALUES ('SHUI_HAO', '水耗');
INSERT INTO test.user_col_comments (column_name, comment) VALUES ('RE_HAO', '热耗');
INSERT INTO test.user_col_comments (column_name, comment) VALUES ('YAN_HAO', '盐耗');
INSERT INTO test.user_col_comments (column_name, comment) VALUES ('OTHER', '其他');
有分数score，score<60返回不及格，score>=60返回及格，score>=80返回优秀
# 有分数score，score<60返回不及格，score>=60返回及格，score>=80返回优秀
SELECT
stu_name,
(CASE WHEN stu_score < 60 THEN '不及格'
WHEN stu_score >= 60 AND stu_score < 80 THEN '及格'
WHEN stu_score >= 80 THEN '优秀'
ELSE '异常' END) AS REMARK
FROM students;
注意：如果你想判断score是否null的情况，WHEN score = null THEN ‘缺席考试’，这是一种错误的写法，正确的写法应为：
CASE WHEN score IS NULL THEN '缺席考试' ELSE '正常' END
现老师要统计班中，有多少男同学，多少女同学，并统计男同学中有几人及格，女同学中有几人及格，要求用一个SQL输出结果。其中stu_sex字段，0表示男生，1表示女生。
SELECT
sum(CASE WHEN STU_SEX = 0 THEN 1 ELSE 0 END) AS MALE_COUNT,
sum(CASE WHEN STU_SEX = 1 THEN 1 ELSE 0 END) AS FEMALE_COUNT,
sum(CASE WHEN STU_SCORE >= 60 AND STU_SEX = 0 THEN 1 ELSE 0 END) AS MALE_PASS,
sum(CASE WHEN STU_SCORE >= 60 AND STU_SEX = 1 THEN 1 ELSE 0 END) AS FEMALE_PASS
FROM
students;
输出结果如下：
注意点：
sum 而不是countTHEN 1 ELSE 0的位置不能改变：否则会有以下效果：sum(CASE WHEN stu_sex = 0 THEN '1' ELSE '0' END) AS '男性',
改变了
sum(CASE WHEN stu_sex = 0 THEN '0' ELSE '1' END) AS '女性'：
现要求统计各个城市，总共使用了多少水耗、电耗、热耗，使用一条SQL语句输出结果
有能耗表如下：其中，E_TYPE表示能耗类型，0表示水耗，1表示电耗，2表示热耗
select e_code,
sum(case when e_type = 0 then e_value else 0 end) as '水耗',
sum(case when e_type = 1 then e_value else 0 end) as '电耗',
sum(case when e_type = 2 then e_value else 0 end) as '热耗'
from energy_test
group by e_code;
输出结果如下：
根据城市用电量多少，计算用电成本。假设电能耗单价分为三档，根据不同的能耗值，使用相应价格计算成本。
当能耗值小于10时，使用P_LEVEL=0时的P_PRICE的值，能耗值大于10小于30使用P_LEVEL=1时的P_PRICE的值…
energy_test 我修改了e_type 为1的值的两条数据的e_value。
select e_code, e_value,
(CASE WHEN e_value <= (SELECT p_limit FROM p_price WHERE p_level = 0)
THEN (SELECT p_price FROM p_price WHERE p_level = 0)
WHEN e_value > (SELECT p_limit FROM p_price WHERE p_level = 0) AND e_value <= (SELECT p_limit FROM p_price WHERE p_level = 1)
THEN (SELECT P_PRICE FROM p_price WHERE P_LEVEL = 1)
WHEN e_value > (SELECT p_limit FROM p_price WHERE p_level = 1) AND e_value <= (SELECT p_limit FROM p_price WHERE p_level = 2)
THEN (SELECT p_price FROM p_price WHERE P_LEVEL = 2) end ) as price
from energy_test
where e_type = 1;
输出结果如下：
行转列中 SUM作用：无用，但是select后得跟聚合函数，不能去掉sum。直接写max或者min也行。
select
max(case when column_name = 'SHI_SHI_CODE' then comment else ''end) as SHI_SHI_CODE_COMMENT,
max(case when column_name = 'SHUI_HAO' then comment else ''end) as SHUI_HAO_COMMENT,
max(case when column_name = 'RE_HAO' then comment else ''end) as RE_HAO_COMMENT,
max(case when column_name = 'YAN_HAO' then comment else ''end) as YAN_HAO_COMMENT,
max(case when column_name = 'OTHER' then comment else '' end) as OTHER_COMMENT
from user_col_comments;
输出结果如下：
