1. CASE WHEN 表達(dá)式有兩種形式

--簡(jiǎn)單Case函數(shù)

CASE sex

WHEN '1' THEN '男'

WHEN '2' THEN '女'

ELSE '其他' END

--Case搜索函數(shù)

CASE

WHEN sex = '1' THEN '男'

WHEN sex = '2' THEN '女'

ELSE '其他' END

2. CASE WHEN 在語(yǔ)句中不同位置的用法

2.1?SELECT?CASE WHEN 用法

SELECT grade, COUNT (CASE WHEN sex = 1 THEN 1

ELSE NULL

END) 男生數(shù),

COUNT (CASE WHEN sex = 2 THEN 1

ELSE NULL

END) 女生數(shù)

FROM students GROUP BY grade;

2.2 WHERE?CASE WHEN 用法

SELECT T2.*, T1.*

FROM T1, T2

WHERE (CASE WHEN T2.COMPARE_TYPE = 'A' AND

T1.SOME_TYPE LIKE 'NOTHING%'

THEN 1

WHEN T2.COMPARE_TYPE != 'A' AND

T1.SOME_TYPE NOT LIKE 'NOTHING%'

THEN 1

ELSE 0

END) = 1

2.3 GROUP BY?CASE WHEN 用法

SELECT

CASE WHEN salary <= 500 THEN '1'

WHEN salary > 500 AND salary <= 600 THEN '2'

WHEN salary > 600 AND salary <= 800 THEN '3'

WHEN salary > 800 AND salary <= 1000 THEN '4'

ELSE NULL END salary_class, -- 別名命名

COUNT(*)

FROM Table_A

GROUP BY

CASE WHEN salary <= 500 THEN '1'

WHEN salary > 500 AND salary <= 600 THEN '2'

WHEN salary > 600 AND salary <= 800 THEN '3'

WHEN salary > 800 AND salary <= 1000 THEN '4'

ELSE NULL END;

3.關(guān)于IF-THEN-ELSE的其他實(shí)現(xiàn)

3.1 DECODE() 函數(shù)

select decode(sex, 'M', 'Male', 'F', 'Female', 'Unknown')

from employees;

只有Oracle提供該函數(shù)，而且不支持ANSI SQL，語(yǔ)法上也沒(méi)CASE WHEN清晰，個(gè)人不推薦使用。

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