Question

postgreSQL问题...我在下面有一个更新查询，它使用子查询的结果更新一列，但是在某些情况下，子查询将返回null，这会在列上引发'not null'约束，我怎么能得到如果子查询返回null，则不更新？

我试过EXISTS，但这似乎只适用于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

coalesce，nvl，ifnull在大多数数据库引擎中都会执行一个条件语句，在这种情况下，当subselect返回null时，取字符串中的第一个非空值，它会将ave_price =设置为自身。

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)

这并不会阻止udpate的请求，但它对数据有类似的影响。

有关合并的更多信息，请参阅：PostgreSQL

要实际阻止更新，您需要在更新中添加where子句并重新执行子查询，例如：

    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

两次逻辑执行子查询会影响性能两次;而coalesce只需执行一次。总有多种方法可以做，并且根据要求，必须选择最适合他们的选项。

Answer 2

使用实际的子查询来代替子查询表达式：

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;

这也会更快。如果表user_id中存在user_sessions的相关比例，而user_stats中没有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;，则以下查询可能会更快（虽然两者都会产生相同的结果）：

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;

第二个版本的要点是尽早排除不相关的行。使用CTE编写的相同查询（稍微更优雅和可读）：

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请注意，虽然在PostgreSQL 8.4中引入了用于SELECT查询的CTE，但数据修改命令的CTE仅为introduced with PostgreSQL 9.1：

允许WITH子句中的数据修改命令（INSERT / UPDATE / DELETE）

postgresql where条件返回至少一个结果

2 个答案: