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)
以上是如何在 MySQL CASE 语句中使用列数据？的详细内容。更多信息请关注PHP中文网其他相关文章！
