我知道查询应该只返回值,因为只有一个客户被验证为学生。
有人能帮我用COUNT(*)
构建select语句吗?
您可以使用WHERE
只对学生进行筛选,然后对他们进行计数`:
SELECT COUNT(*) AS StudentCount
FROM Customer
WHERE Student = 1
或者使用COUNT
与CASE
相结合(可能较慢):
SELECT COUNT(CASE WHEN Student = 1 THEN 1 END) AS StudentCount
FROM Customer
甚至MySQL
:
SELECT SUM(Student = 1) AS StudentCount
FROM Customer