PostgreSQL：更新時忽略了左外部自聯接

Question

我想更新列leaf_category和TRUE，其中類別不是父類別。它可以作爲一個select語句：

select 
    c1.id, c1.name, c1.slug, c1.level, c2.parent_id, c2.name, c2.slug, c2.level 
from 
    catalog_category c1 
left outer join 
    catalog_category c2 on 
    (c1.id = c2.parent_id) 
where 
    c2.parent_id is null; 

然而，相應的UPDATE集所有列TRUE。

update catalog_category 
set leaf_category = True 
from 
    catalog_category c1 
left outer join 
    catalog_category c2 on 
    (c1.id = c2.parent_id) 
where 
    c2.parent_id is null; 

是否可以使用UPDATE？

Answer 1

你只是缺少一個連接WHERE子句：

UPDATE catalog_category 
SET leaf_category = TRUE 
FROM catalog_category c1 
LEFT join catalog_category c2 ON c1.id = c2.parent_id 
WHERE catalog_category.id = c1.id 
AND c2.parent_id IS NULL; 

這種形式NOT EXISTS是可能更快，同時做相同的：

UPDATE catalog_category 
SET leaf_category = TRUE 
WHERE NOT EXISTS (
    SELECT * 
    FROM catalog_category c 
    WHERE c.parent_id = catalog_category.id 
    );