CASE 表达式可以实现类似“IF…ELSE…THEN”的逻辑而不用调用子程序。CASE 表达式有两种使用方法，简单的和带搜索条件的。

• 示例：在查询中使用简单的 CASE 表达式，将国家代码缩写翻译为全称。

obclient> CREATE TABLE t_case(id number NOT NULL PRIMARY KEY, abbr varchar(5));
Query OK, 0 rows affected (0.06 sec)
obclient> INSERT INTO t_case(id, abbr) VALUES (1,'US'),(2,'UK'),(3,'CN'),(4,'JP');
Query OK, 4 rows affected (0.03 sec)
Records: 4  Duplicates: 0  Warnings: 0
obclient>
obclient> SELECT id, abbr,
     CASE abbr
         WHEN 'US' THEN 'America'
         WHEN 'UK' THEN 'English'
         WHEN 'CN' THEN 'China'
     ELSE 'UNKOWN'
     END full_name
 FROM t_case ;
+----+------+-----------+
| ID | ABBR | FULL_NAME |
+----+------+-----------+
|  1 | US   | America   |
|  2 | UK   | English   |
|  3 | CN   | China     |
|  4 | JP   | UNKOWN    |
+----+------+-----------+
4 rows in set (0.00 sec)
obclient>

• 示例：在查询中使用带搜索条件的 CASE 表达式

obclient> CREATE TABLE t_case2(id number NOT NULL PRIMARY KEY, c_date date );
Query OK, 0 rows affected (0.07 sec)
obclient> INSERT INTO t_case2(id,c_date) 
VALUES (1,to_date('2019-03-01','yyyy-mm-dd'))
     ,(2,to_date('2019-05-08','yyyy-mm-dd'))
     ,(3,to_date('2019-07-07','yyyy-mm-dd'))
     ,(4,to_date('2019-10-11','yyyy-mm-dd'))
     ,(5,to_date('2019-12-12','yyyy-mm-dd'))
     ,(6,to_date('2020-01-05','yyyy-mm-dd'));
Query OK, 6 rows affected (0.01 sec)
Records: 6  Duplicates: 0  Warnings: 0
obclient> SELECT id, c_date,
     CASE
         WHEN months_between(sysdate, c_date) > 12 THEN 'More than one year ago'
         WHEN months_between(sysdate, c_date) > 9 THEN 'More than three quarters ago'
         WHEN months_between(sysdate, c_date) > 6 THEN 'More than half a year ago'
         WHEN months_between(sysdate, c_date) > 3 THEN 'More than a quarter ago'
         WHEN months_between(sysdate, c_date) >= 0 THEN 'Within a quarter'
         ELSE 'Illegal'
     END "Duration"
 FROM t_case2;
+----+-----------+------------------------------+
| ID | C_DATE    | Duration                     |
+----+-----------+------------------------------+
|  1 | 01-MAR-19 | More than one year ago       |
|  2 | 08-MAY-19 | More than three quarters ago |
|  3 | 07-JUL-19 | More than half a year ago    |
|  4 | 11-OCT-19 | More than a quarter ago      |
|  5 | 12-DEC-19 | More than a quarter ago      |
|  6 | 05-JAN-20 | Within a quarter             |
+----+-----------+------------------------------+
6 rows in set (0.00 sec)
obclient>