Introduction to Oracle CASE WHEN usage
1. The CASE WHEN expression has two forms
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--Simple Case function
CASE sex
WHEN '1' THEN '男'
WHEN '2' THEN '女'
ELSE 'other' END
--Case search function
CASE
WHEN sex = '1' THEN '男'
WHEN sex = '2' THEN '女'
ELSE 'other' END
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2. The usage of CASE WHEN in different positions in the statement
2.1 SELECT CASE WHEN usage
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SELECT grade, COUNT (CASE WHEN sex = 1 THEN 1 /*sex 1 is a boy, 2 girls*/
ELSE NULL
END) number of boys,
COUNT (CASE WHEN sex = 2 THEN 1
ELSE NULL
END) Number of girls
FROM students GROUP BY grade;
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2.3 WHERE CASE WHEN 用法
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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
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2.4 GROUP BY CASE WHEN 用法
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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, -- alias name
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;
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3. Other implementations of IF-THEN-ELSE
3.1 DECODE() function
select decode(sex, 'M', 'Male', 'F', 'Female', 'Unknown')
from employees;
It seems that only Oracle provides this function, and it does not support ANSI SQL, and the syntax is not clear in CASE WHEN. Personally, it is not recommended to use it.
3.2 Special implementation in WHERE
SELECT T2.*, T1.*
FROM T1, T2
WHERE (T2.COMPARE_TYPE = 'A' AND T1.SOME_TYPE LIKE 'NOTHING%')
OR
(T2.COMPARE_TYPE != 'A' AND T1.SOME_TYPE NOT LIKE 'NOTHING%')
This method is also used in special cases, pay more attention to the logic and do not make mistakes.