CASE表达式可以在SQL中实现if-then-else型的逻辑，而不必使用PL/SQL。CASE的工作方式与DECODE()类似，但应该使用CASE,因为它与ANSI兼容。

CASE有两种表达式:

1. 简单CASE表达式,使用表达式确定返回值.

语法:

CASE search_expression
WHEN expression1 THEN result1
WHEN expression2 THEN result2
...
WHEN expressionN THEN resultN
ELSE default_result
END

例:

select product_id,product_type_id,
case product_type_id
when 1 then 'Book'
when 2 then 'Video'
when 3 then 'DVD'
when 4 then 'CD'
else 'Magazine'
end
from products

结果:

PRODUCT_ID PRODUCT_TYPE_ID CASEPROD
---------- --------------- --------
         1               1                             Book
         2               1                             Book
         3               2                              Video
         4               2                             Video
         5               2                             Video
         6               2                            Video
         7               3                             DVD
         8               3                             DVD
         9               4                              CD
        10               4                               CD
        11               4                             CD
        12                                            Magazine

12 rows selected.

2. 搜索CASE表达式,使用条件确定返回值.

语法:

CASE

WHEN condition1 THEN result1
WHEN condistion2 THEN result2
...
WHEN condistionN THEN resultN
ELSE default_result
END

例:

select product_id,product_type_id,
case
when product_type_id=1 then 'Book'
when product_type_id=2 then 'Video'
when product_type_id=3 then 'DVD'
when product_type_id=4 then 'CD'
else 'Magazine'
end
from products