SQL中的CASE WHEN用法小結(jié)

更新時間：2025年03月19日 10:52:55 作者：m0_74824123

文章詳細(xì)介紹了SQL中的CASEWHEN函數(shù)及其用法,包括簡單CASEWHEN和CASEWHEN條件表達(dá)式兩種形式,并通過多個實(shí)際場景展示了如何使用CASEWHEN進(jìn)行數(shù)據(jù)判斷和條件統(tǒng)計(jì),感興趣的朋友跟隨小編一起看看吧

一、簡單CASE WHEN函數(shù)：

CASE SCORE WHEN 'A' THEN '優(yōu)' ELSE '不及格' END
# 使用 IF 函數(shù)進(jìn)行替換
IF(SCORE = 'A', '優(yōu)', '不及格')

THEN后邊的值與ELSE后邊的值類型應(yīng)一致，否則會報(bào)錯。
如下：
CASE SCORE WHEN ‘A’ THEN ‘優(yōu)’ ELSE 0 END’優(yōu)’和0數(shù)據(jù)類型不一致則報(bào)錯：
[Err] ORA-00932: 數(shù)據(jù)類型不一致: 應(yīng)為 CHAR, 但卻獲得 NUMBER

簡單CASE WHEN函數(shù)只能應(yīng)對一些簡單的業(yè)務(wù)場景，而CASE WHEN條件表達(dá)式的寫法則更加靈活。

二、CASE WHEN條件表達(dá)式函數(shù)

類似JAVA中的IF ELSE語句。

格式：

CASE WHEN condition THEN result
[WHEN...THEN...]
ELSE result
END

SQL語言演示：

CASE 
	 WHEN SCORE = 'A' THEN '優(yōu)'
     WHEN SCORE = 'B' THEN '良'
     WHEN SCORE = 'C' THEN '中' 
     ELSE '不及格' END
# 等同于
CASE score
    WHEN 'A' THEN '優(yōu)'
    WHEN 'B' THEN '良'
    WHEN 'C' THEN '中'
    ELSE '不及格' END

condition是一個返回布爾類型的表達(dá)式，
如果表達(dá)式返回true，則整個函數(shù)返回相應(yīng)result的值，
如果表達(dá)式皆為false，則返回ElSE后result的值，如果省略了ELSE子句，則返回NULL。

三、常用場景

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', '曉峰', 0, 45);
INSERT INTO students (stu_code, stu_name, stu_sex, stu_score) VALUES ('xh', '小紅', 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', '小時', 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表示能耗類型，0表示水耗，1表示電耗，2表示熱耗
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 '價格',
    p_level int           null comment '等級',
    p_limit int           null comment '閾值'
)
    comment '電能耗單價表';

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 '列的備注'
);

user_col_comments 表的DML

INSERT INTO test.user_col_comments (column_name, comment) VALUES ('SHI_SHI_CODE', '設(shè)施編號');
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', '熱耗');
INSERT INTO test.user_col_comments (column_name, comment) VALUES ('YAN_HAO', '鹽耗');
INSERT INTO test.user_col_comments (column_name, comment) VALUES ('OTHER', '其他');

場景1：不同狀態(tài)展示為不同的值

有分?jǐn)?shù)score，score<60返回不及格，score>=60返回及格，score>=80返回優(yōu)秀

# 有分?jǐn)?shù)score，score<60返回不及格，score>=60返回及格，score>=80返回優(yōu)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 '優(yōu)秀'
        ELSE '異常' END) AS REMARK
FROM students;

注意：如果你想判斷score是否null的情況，WHEN score = null THEN ‘缺席考試’，這是一種錯誤的寫法，正確的寫法應(yīng)為：
CASE WHEN score IS NULL THEN '缺席考試' ELSE '正常' END

場景2：統(tǒng)計(jì)不同狀態(tài)下的值

現(xiàn)老師要統(tǒng)計(jì)班中，有多少男同學(xué)，多少女同學(xué)，并統(tǒng)計(jì)男同學(xué)中有幾人及格，女同學(xué)中有幾人及格，要求用一個SQL輸出結(jié)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;

輸出結(jié)果如下：

注意點(diǎn)：

用的是：sum?而不是count
THEN 1 ELSE 0的位置不能改變：否則會有以下效果：

sum(CASE WHEN stu_sex = 0 THEN '1' ELSE '0' END) AS '男性',
改變了 
sum(CASE WHEN stu_sex = 0 THEN '0' ELSE '1' END) AS '女性'：

字符 ‘0’ 和數(shù)值 0，使用都是一樣的

場景3：配合聚合函數(shù)做統(tǒng)計(jì)

現(xiàn)要求統(tǒng)計(jì)各個城市，總共使用了多少水耗、電耗、熱耗，使用一條SQL語句輸出結(jié)果
有能耗表如下：其中，E_TYPE表示能耗類型，0表示水耗，1表示電耗，2表示熱耗

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 '電耗',
       sum(case when e_type = 2 then e_value else 0 end) as '熱耗'
from energy_test
group by e_code;

輸出結(jié)果如下：

場景4：CASE WHEN中使用子查詢

根據(jù)城市用電量多少，計(jì)算用電成本。假設(shè)電能耗單價分為三檔，根據(jù)不同的能耗值，使用相應(yīng)價格計(jì)算成本。
當(dāng)能耗值小于10時，使用P_LEVEL=0時的P_PRICE的值，能耗值大于10小于30使用P_LEVEL=1時的P_PRICE的值…

energy_test 我修改了e_type 為1的值的兩條數(shù)據(jù)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;

輸出結(jié)果如下：

場景5：經(jīng)典行轉(zhuǎn)列，結(jié)合max聚合函數(shù)

行轉(zhuǎn)列中 SUM作用：無用，但是select后得跟聚合函數(shù)，不能去掉sum。直接寫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;

輸出結(jié)果如下：