SQL之CASE WHEN用法小结
作者:遨游在知识的海洋里无法自拔
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
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:
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