PostgreSQL：跨多个列（与其他选择一起）计算不同的值

PostgreSQL中的简单SQL语句：

select class_offerings.semester_code as SC, class_offerings.class_code as CC,
class_offerings.class_name as CN, class_offerings.teacher_name as TN, 
count(distinct class_enrollment.semester_code, 
class_enrollment.class_code) as num_enrolled, --wrong
class_enrollment.maximum_capacity as max_enrollment 
from class_offerings natural join class_enrollment;

PostgreSQL不支持此语法。我该怎么做呢？预期结果：

SC | CC | CN | TN | num_enrolled | max_enrollment

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切勿使用

自然连接

。这是一个令人厌恶的问题，因为它依赖于具有相同名称的列。它甚至没有使用正确声明的外键关系

表别名使查询更易于编写和读取。也许你打算：

select o.semester_code as SC, o.class_code as CC, o.class_name as CN, o.teacher_name as TN, 
       count(*) as num_enrolled, --wrong
from class_offerings o join
     class_enrollment e
     on c.? = e.?
group by o.semester_code, o.class_code, o.class_name, o.teacher_name;

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在

？

所在的

子句上的
中放置联接列。
编辑您的问题，并提供示例数据和所需结果。