SUM 最重要還是 where clause select + where = filter select + sum() + where = satisfy 以前細個學又話叫 filter … 其實我覺得叫 satisfy 好d

又所以 我發覺 sum() subquery in select clause, group by 係唔需要:

select a, b, (select sum(xxx) where [CONDITION] 這裡放 group by 無用) as yyy, …

select …(select sum(xxx) where sth)只能回一個 single row , 唔係就會 error, 所以個 where clause 要俾夠 info 去砌到得翻一個 rowgroup by 本身 for multiple row 去教 table 點 group … 但 single row 其實就無必要

我開頭以為 sum() 既 logic 係 define 係 group by which is 錯既 … 但原來都係放係 where 到

ref:

https://stackoverflow.com/questions/20557899/how-does-subquery-in-select-statement-work-in-oracle

就係可以同阿媽互動既 小 table

search more by keyword: Subqueries in SELECT Statements

https://en.wikipedia.org/wiki/Relational_algebra

用loop 就叫 imperative
用 map 就叫 declarative
真係唔講唔知
最好就睇埋咩叫 how to write flat code