PostgreSQL：跨多个列计算唯一性（其他选择旁边）

Question

PostgreSQL中的一个简单的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。 我该怎么办呢？ 预期结果：

SC | CC | CN | TN | num_enrolled | max_enrollment

Answer 1

切勿使用natural join 。 这是可憎的，因为它取决于具有相同名称的列。 它甚至不使用正确声明的外键关系。

表别名使查询更易于编写和阅读。 也许您打算：

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;

将用于连接的列放在on子句中，其中? 是。