登录注册窗口（一）-CFANZ编程社区

3、IFNULL() 函数，如果第一个参数为 NULL，返回第二个参数，否则返回第一个参数。

4、NULLIF() 函数根据两个参数是否相等决定返回 NULL 还是第一个参数。

1、`CASE` 语句用于流程控制中的多分支情况。

SELECT
    weekday_index AS `Weekday Index`,
    CASE weekday_index
        WHEN 0 THEN 'Monday'
        WHEN 1 THEN 'Tuesday'
        WHEN 2 THEN 'Wednesday'
        WHEN 3 THEN 'Thursday'
        WHEN 4 THEN 'Friday'
        WHEN 5 THEN 'Saturday'
        WHEN 6 THEN 'Sunday'
        ELSE 'Error'
    END AS `Weekday Name`
FROM
    test_case_weekday;


+---------------+--------------+
| Weekday Index | Weekday Name |
+---------------+--------------+
|             0 | Monday       |
|             1 | Tuesday      |
|             2 | Wednesday    |
|             3 | Thursday     |
|             4 | Friday       |
|             5 | Saturday     |
|             6 | Sunday       |
+---------------+--------------+

SELECT
    weekday_index AS `Weekday Index`,
    CASE
        WHEN weekday_index = 0 THEN 'Monday'
        WHEN weekday_index = 1 THEN 'Tuesday'
        WHEN weekday_index = 2 THEN 'Wednesday'
        WHEN weekday_index = 3 THEN 'Thursday'
        WHEN weekday_index = 4 THEN 'Friday'
        WHEN weekday_index = 5 THEN 'Saturday'
        WHEN weekday_index = 6 THEN 'Sunday'
        ELSE 'Error'
    END AS `Weekday Name`
FROM
    test_case_weekday;

2、`IF()` 函数根据测试条件是否为真分别返回指定的值。

SELECT IF(1 > 2, 'NO', 'YES'), IF(2 > 1, 'NO', 'YES');


+------------------------+------------------------+
| IF(1 > 2, 'NO', 'YES') | IF(2 > 1, 'NO', 'YES') |
+------------------------+------------------------+
| YES                    | NO                     |
+------------------------+------------------------+

3、`IFNULL()` 函数，如果第一个参数为 `NULL`，返回第二个参数，否则返回第一个参数。

SELECT
    IFNULL(NULL, 'It is NULL'),
    IFNULL('I am not NULL', 'I am NULL');


+----------------------------+--------------------------------------+
| IFNULL(NULL, 'It is NULL') | IFNULL('I am not NULL', 'I am NULL') |
+----------------------------+--------------------------------------+
| It is NULL                 | I am not NULL                        |
+----------------------------+--------------------------------------+

4、`NULLIF()` 函数根据两个参数是否相等决定返回 `NULL` 还是第一个参数。

SELECT
    NULLIF(1, 1),
    NULLIF(1, 2);


+--------------+--------------+
| NULLIF(1, 1) | NULLIF(1, 2) |
+--------------+--------------+
|         NULL |            1 |
+--------------+--------------+