MySQL CASE语句在什么情况下返回NULL？

mysql> Select CASE 100
    -> WHEN 150 THEN 'It is matched'
    -> WHEN 200 THEN 'It is not matched'
    -> END As 'It Returns NULL';
+-----------------+
| It Returns NULL |
+-----------------+
| NULL            |
+-----------------+
1 row in set (0.00 sec)

mysql> Select SUM(CASE WHEN country = 'USA' THEN 1 ELSE 0 END) AS USA,
    -> SUM(CASE WHEN country = 'UK' THEN 1 ELSE 0 END) AS UK,
    -> SUM(CASE WHEN country = 'INDIA' THEN 1 ELSE 0 END) AS INDIA,
    -> SUM(CASE WHEN country = 'Russia' THEN 1 ELSE 0 END) AS Russia,
    -> SUM(CASE WHEN country = 'France' THEN 1 ELSE 0 END) AS France,
    -> SUM(CASE WHEN country = 'NZ' THEN 1 ELSE 0 END) AS NZ,
    -> SUM(CASE WHEN country = 'Australia' THEN 1 ELSE 0 END) AS Australia,
    -> SUM(CASE WHEN country = 'WI' THEN 1 END) AS WI
    -> From Students;
+------+------+-------+--------+--------+------+-----------+------+
| USA  | UK   | INDIA | Russia | France | NZ   | Australia | WI   |
+------+------+-------+--------+--------+------+-----------+------+
| 2    | 1    | 2     | 1      | 1      | 1    | 1         | NULL |
+------+------+-------+--------+--------+------+-----------+------+
1 row in set (0.00 sec)