使用 CTE 或连接所有表哪个更好

Question

我有一个包含大量左连接的查询，并且我只检索了大多数左连接的一列，是对每个表使用 CTE 来仅检索需要的内容更好还是更好？ 还是继续连接所有这些表更好，即使我每个左连接只需要一个表。

Answer 1

您想要从表中获得多少列并不重要。对于数据库管理系统

select t1.a, t2.b 
from t1
left join t2 using (x)

与

相同

select dt1.a, dt2.b 
from (select x, a from t1) dt1
left join (select x, b from t2) dt2 using (x)

或

with dt1 as (select x, a from t1) 
     dt2 as (select x, b from t2)
select dt1.a, dt2.b 
from dt1
left join dt2 using (x)

所以，保持简单。只需加入表格即可。这使得查询易于阅读，从而易于维护。

但是，一旦聚合数据，您通常会希望在加入之前进行聚合：

with upv as (select post_id, count(*) as total from upvotes group by post_id) 
     dnv as (select post_id, count(*) as total from downvotes group by post_id)
select
  post_id,
  upv.total as total_upvotes,
  dnv.total as total_downvotes
from upv
join dnv using (x)
order by post_id;