postgresql where條件返回至少一個結果

Question

postgreSQL問題...我有一個更新查詢，它使用來自子查詢的結果更新列，但是在某些情況下，子查詢將返回null，從而引發'not null'約束在列上，如果子查詢返回null，我怎麼才能更新它？

我已經嘗試過EXISTS，但這似乎只適用於WHERE子句？

UPDATE user_stats as stats 
SET ave_price = (
    SELECT AVG(l.price) 
    FROM lengths as l, user_sessions as us 
    WHERE l.product_type = 'car' 
    AND l.session_id = us.session_id 
    AND stats.user_id = us.user_id 
) 

Answer 1

聚結，NVL，IFNULL在大多數數據庫引擎將做一個有條件的聲明說，拿在這種情況下，字符串中的第一個非空值時，再選擇返回null它將設置ave_price =本身。

UPDATE user_stats as stats 
SET ave_price = coalesce((
    SELECT AVG(l.price) 
    FROM lengths as l, user_sessions as us 
    WHERE l.product_type = 'car' 
    AND l.session_id = us.session_id 
    AND stats.user_id = us.user_id 
),ave_price) 

這並不妨礙udpate的要求，但它對數據有類似的影響。

有關COALESCE查看更多資訊：PostgreSQL

要真正防止更新你會needto上的更新添加一個WHERE子句和重新執行子查詢，如：

UPDATE user_stats as stats 
    SET ave_price = (
     SELECT AVG(l.price) 
     FROM lengths as l, user_sessions as us 
     WHERE l.product_type = 'car' 
     AND l.session_id = us.session_id 
     AND stats.user_id = us.user_id) 
WHERE (SELECT AVG(l.price) 
     FROM lengths as l, user_sessions as us 
     WHERE l.product_type = 'car' 
     AND l.session_id = us.session_id 
     AND stats.user_id = us.user_id) is not null 

邏輯上執行子查詢兩次會對性能產生兩次影響;而合併只需要執行一次。總是有多種方式來完成任務，並且根據要求，必須選擇哪種選項最適合他們。

Answer 2

使用的實際子查詢的，而不是一個子查詢表達式選擇：

UPDATE user_stats s 
SET ave_price = x.ave_price 
FROM (
    SELECT user_id 
      ,avg(l.price) AS ave_price 
    FROM lengths l 
    JOIN user_sessions us ON us.session_id = l.session_id 
    WHERE l.product_type = 'car' 
    GROUP BY us.user_id 
    HAVING avg(l.price) IS NOT NULL 
    ) x 
WHERE x.user_id = s.user_id; 

這將是更快了。 如果你有user_id相關的比例存在於表user_sessions，但不是在user_stats，那麼下面的查詢可能會更快（雖然兩者產量在每一種情況下相同的結果）：

UPDATE user_stats s 
SET ave_price = x.ave_price 
FROM (
    SELECT user_id 
      ,avg(l.price) AS ave_price 
    FROM lengths l 
    JOIN user_stats usr USING (user_id) 
    JOIN user_sessions us ON us.session_id = l.session_id 
    WHERE l.product_type = 'car' 
    GROUP BY us.user_id 
    HAVING avg(l.price) IS NOT NULL 
    ) x 
WHERE x.user_id = s.user_id; 

的點第二個版本是儘早排除不相關的行。 的CTE（有點更優雅和可讀的）寫入相同的查詢：

WITH x AS (
    SELECT user_id 
      ,avg(l.price) AS ave_price 
    FROM lengths l 
    JOIN user_stats usr USING (user_id) 
    JOIN user_sessions us ON us.session_id = l.session_id 
    WHERE l.product_type = 'car' 
    GROUP BY us.user_id 
    HAVING avg(l.price) IS NOT NULL 
    ) 
UPDATE user_stats s 
SET ave_price = x.ave_price 
FROM x 
WHERE x.user_id = s.user_id; 

注意，而CTE爲SELECT查詢用的PostgreSQL 8.4引入，CTE爲數據修改命令只有introduced with PostgreSQL 9.1：

在WITH子句