SQL中的CASE WHEN用法小结_MsSqlserver

一、简单case when函数：

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。

二、case when条件表达式函数

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。

三、常用场景

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');

场景1：不同状态展示为不同的值

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

场景2：统计不同状态下的值

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：

用的是：sum?而不是count
then 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'：

字符 ‘0’ 和数值 0，使用都是一样的

场景3：配合聚合函数做统计

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：

场景4：case when中使用子查询

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：

场景5：经典行转列，结合max聚合函数

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：