如果子查詢返回空，則取消更新中的列

Question

我有以下SQL查詢

UPDATE container  SET member_containers = members
FROM 
(
SELECT inter_container_membership.owning_container_id AS owner_cont
      ,array_to_string(array_agg(inter_container_membership.member_container_id),',') AS members
FROM inter_container_membership 
GROUP BY inter_container_membership.owning_container_id) v
WHERE container.id = owner_cont; 

問題是，如果sub-query不返回任何內容，則member_containers將不會更新，如果是這種情況，我需要設置為 null，我嘗試使用EXISTS子句但它沒有用。

UPDATE container  SET member_containers = 
     CASE WHEN EXISTS (
          SELECT * FROM inter_container_membership
          WHERE container.id  = inter_container_membership.owning_container_id
          ) THEN members
     ELSE NULL END
FROM  (
SELECT inter_container_membership.owning_container_id AS owner_cont
      ,array_to_string(array_agg(inter_container_membership.member_container_id),',') AS members
FROM inter_container_membership 
GROUP BY inter_container_membership.owning_container_id) v
WHERE container.id = owner_cont;

所以現在我想在更新之前取消member_containers ，但我找不到這樣做的方法。

Answer 1

要在子查詢未找到任何內容的情況下將列設置為 NULL，您可以使用相關子查詢而不是連接：

UPDATE container c
SET    member_containers = (
   SELECT string_agg(member_container_id, ',')
   FROM   inter_container_membership i
   WHERE  i.owning_container_id = c.id
   GROUP  BY owning_container_id
   );

通常，這種查詢是錯誤的，因為如果未找到任何內容，則不應取消該列。 但你似乎正是想要那個。
此外，相關子查詢往往很慢。 考慮添加這個左連接（做同樣的事情）：

UPDATE container c
SET    member_containers = i.members
FROM   container c1
LEFT   JOIN (
   SELECT owning_container_id
        , string_agg(member_container_id, ',') AS members
   FROM   inter_container_membership 
   GROUP  BY 1
   ) i ON i.owning_container_id = c1.id
WHERE  c.id = c1.id;

還使用string_agg()而不是array_to_string(array_agg(...)) 。 需要 Postgres 9.0+。

Answer 2

使用 CTE：

WITH m AS (
  SELECT icm.owning_container_id AS owner_cont,
         array_to_string(array_agg(icm.member_container_id),',') AS members
  FROM inter_container_membership icm
  GROUP BY icm.owning_container_id)
UPDATE container SET member_containers = m.members
WHERE id = m.owner_cont;