要理解它，请考虑“学生”表中的数据，如下所示 -
mysql> Select * from Students;
+----+-----------+-----------+----------+----------------+
| id | Name | Country | Language | Course |
+----+-----------+-----------+----------+----------------+
| 1 | Francis | UK | English | Literature |
| 2 | Rick | USA | English | History |
| 3 | Correy | USA | English | Computers |
| 4 | Shane | France | French | Computers |
| 5 | Validimir | Russia | Russian | Computers |
| 6 | Steve | Australia | English | Geoinformatics |
| 7 | Rahul | India | Hindi | Yoga |
| 8 | Harshit | India | Hindi | Computers |
| 9 | Harry | NZ | English | Electronics |
+----+-----------+-----------+----------+----------------+
9 rows in set (0.00 sec)现在，假设我们想知道有多少学生属于美国、英国、新西兰、印度、俄罗斯、法国等，那么我们可以在 CASE 语句中使用“国家/地区”列，如下所示-
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
-> From Students;
+------+------+-------+--------+--------+------+-----------+
| USA | UK | INDIA | Russia | France | NZ | Australia |
+------+------+-------+--------+--------+------+-----------+
| 2 | 1 | 2 | 1 | 1 | 1 | 1 |
+------+------+-------+--------+--------+------+-----------+
1 row in set (0.07 sec)
