如何避免`select suminto`在PostgreSQL中将目标变量设置为null

问题描述

此select sum()是触发功能的一部分，并且可以正常唤醒，

-- loop over all order lines without grouping
select
  sum(tp.pack_volume),sum(tp.pack_mass),sum(tl.qty_ordered),sum(tl.qty_fulfilled)
into
  new.pack_expected_volume,new.pack_expected_mass,new.qty_ordered,new.qty_fulfilled
from 
  order_lines tl
join 
  product tp on tl.product_id = tp.id
where
  tl.order_id = new.id;

-- avoid nulls if no rows in of_order_lines
new.pack_expected_volume = coalesce (new.pack_expected_volume,0);
new.pack_expected_mass = coalesce (new.pack_expected_mass,0);
new.qty_ordered = coalesce (new.qty_ordered,0);
new.qty_fulfilled = coalesce (new.qty_fulfilled,0);

但是，我必须添加那些可怕的coalesce行以检查所有汇总结果都不为空。当表order_lines中没有行时，肯定会发生这种情况。

有人知道一种更优雅/干净的方法，而无需在select之后重新检查null吗？

解决方法

只需在SELECT列表中这样做：

select
  coalesce(sum(tp.pack_volume),0),coalesce(sum(tp.pack_mass),coalesce(sum(tl.qty_ordered),coalesce(sum(tl.qty_fulfilled),0)
into
  new.pack_expected_volume,new.pack_expected_mass,new.qty_ordered,new.qty_fulfilled
from 
  order_lines tl
...