[英]PostgreSQL: Counting Distinct Across Multiple Columns (Alongside Other Selections)
PostgreSQL中的一個簡單的SQL語句:
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不支持此語法。 我該怎么辦呢? 預期結果:
SC | CC | CN | TN | num_enrolled | max_enrollment
切勿使用natural join
。 這是可憎的,因為它取決於具有相同名稱的列。 它甚至不使用正確聲明的外鍵關系。
表別名使查詢更易於編寫和閱讀。 也許您打算:
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;
將用於連接的列放在on
子句中,其中?
是。
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.