CASE 语句
CASE selector
WHEN value1 THEN action1;
WHEN value2 THEN action2;
WHEN value3 THEN action3;
…..
ELSE actionN;
END CASE;
CASE表达式
DECLARE
temp VARCHAR2(10);
v_num number;
BEGIN
v_num := &i;
temp := CASE v_num
WHEN 0 THEN 'Zero'
WHEN 1 THEN 'One'
WHEN 2 THEN 'Two'
ELSE
NULL
END;
dbms_output.put_line('v_num = '||temp);
END;
/
CASE搜索语句
CASE
WHEN (boolean_condition1) THEN action1;
WHEN (boolean_condition2) THEN action2;
WHEN (boolean_condition3) THEN action3;
……
ELSE actionN;
END CASE;
CASE搜索表达式
DECLARE
a number := 20;
b number := -40;
tmp varchar2(50);
BEGIN
tmp := CASE
WHEN (a>b) THEN 'A is greater than B'
WHEN (a<b) THEN 'A is less than B'
ELSE
'A is equal to B'
END;
dbms_output.put_line(tmp);
END;
/
SELECT CASE WHEN 的使用方法
select 与 case结合使用最大的优点有两点,一是在显示查询结果时能够灵活的组织格式,二是有效避免了多次对同一个表或几个表的訪问。以下举个简单的样例来说明。比如表 students(id, name ,birthday, sex, grade),要求按每一个年级统计男生和女生的数量各是多少,统计结果的表头为,年级,男生数量,女生数量。假设不用select case when,为了将男女数量并列显示,统计起来非常麻烦,先确定年级信息,再依据年级取男生数和女生数,并且非常easy出错。用select case when写法例如以下:
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;