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标题: oracle case when 语句的用法详解 [打印本页]

作者: 张兴康 时间: 2020-4-15 15:11
标题: oracle case when 语句的用法详解
1. CASE WHEN 表达式有两种形式
--简单Case函数
CASE sex
WHEN '1' THEN '男'
WHEN '2' THEN '女'
ELSE '其他' END
--Case搜索函数
CASE
WHEN sex = '1' THEN '男'
WHEN sex = '2' THEN '女'
ELSE '其他' END

2. CASE WHEN 在语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    /*sex 1为男生，2位女生*/
                     ELSE NULL
                     END) 男生数,
            COUNT (CASE WHEN sex = 2 THEN 1
                     ELSE NULL
                     END) 女生数
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, -- 别名命名
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.关于IF-THEN-ELSE的其他实现
3.1 DECODE() 函数
select decode(sex, 'M', 'Male', 'F', 'Female', 'Unknown')
from employees;

3.2 在WHERE中特殊实现
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%')

作者: 张兴康 时间: 2020-4-16 14:12

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