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。
    1. obclient> CREATE TABLE t_case(id number NOT NULL PRIMARY KEY, abbr varchar(5));
    2. Query OK, 0 rows affected (0.08 sec)
    4. obclient> INSERT INTO t_case(id, abbr) VALUES (1,'US'),(2,'UK'),(3,'CN'),(4,'JP');
    5. Query OK, 4 rows affected (0.02 sec)
    6. Records: 4  Duplicates: 0  Warnings: 0
    8. obclient>
    9. obclient> SELECT id, abbr,
    10.     CASE abbr
    11.          WHEN 'US' THEN 'America'
    12.          WHEN 'UK' THEN 'English'
    13.          WHEN 'CN' THEN 'China'
    14.     ELSE 'UNKOWN'
    15.     END full_name
    16.  FROM t_case ;
    18. +----+------+-----------+
    19. | id | abbr | full_name |
    20. +----+------+-----------+
    21. |  1 | US   | America   |
    22. |  2 | UK   | English   |
    23. |  3 | CN   | China     |
    24. |  4 | JP   | UNKOWN    |
    25. +----+------+-----------+
    26. 4 rows in set (0.00 sec)
    28. obclient>
    • 示例：在查询中使用带搜索条件的 CASE 表达式
    1. obclient> DROP TABLE IF EXISTS t_case2;
    2. Query OK, 0 rows affected (0.02 sec)
    4. obclient> CREATE TABLE t_case2(id number NOT NULL PRIMARY KEY, c_date date );
    5. Query OK, 0 rows affected (0.14 sec)
    7. obclient> INSERT INTO t_case2(id,c_date) 
    8. VALUES (1,'2019-03-01')
    9.      ,(2,'2019-05-08')
    10.      ,(3,'2019-07-07')
    11.      ,(4,'2019-10-11')
    12.      ,(5,'2019-12-12')
    13.      ,(6,'2020-01-05');
    14. Query OK, 6 rows affected (0.01 sec)
    15. Records: 6  Duplicates: 0  Warnings: 0
    17. obclient>
    18. obclient> SELECT id, c_date,
    19.      CASE
    20.          WHEN datediff(now(), c_date) > 12*30 THEN 'More than one year ago'
    21.          WHEN datediff(now(), c_date) > 9*30 THEN 'More than three quarters ago'
    22.          WHEN datediff(now(), c_date) > 6*30 THEN 'More than half a year ago'
    23.          WHEN datediff(now(), c_date) > 3*30 THEN 'More than a quarter ago'
    24.          WHEN datediff(now(), c_date) >= 0 THEN 'Within a quarter'
    25.          ELSE 'Illegal'
    26.      END "Duration"
    27.  FROM t_case2;
    28. +----+------------+------------------------------+
    29. | id | c_date     | Duration                     |
    30. +----+------------+------------------------------+
    31. |  1 | 2019-03-01 | More than one year ago       |
    32. |  2 | 2019-05-08 | More than three quarters ago |
    33. |  3 | 2019-07-07 | More than three quarters ago |
    34. |  4 | 2019-10-11 | More than a quarter ago      |
    35. |  5 | 2019-12-12 | More than a quarter ago      |
    36. |  6 | 2020-01-05 | Within a quarter             |
    37. +----+------------+------------------------------+
    38. 6 rows in set (0.01 sec)