如果返回值为 null，postgresql 返回 0

Question

我有一个返回 avg(price) 的查询

  select avg(price)
  from(
      select *, cume_dist() OVER (ORDER BY price desc) from web_price_scan
      where listing_Type='AARM'
        and u_kbalikepartnumbers_id = 1000307
        and (EXTRACT(Day FROM (Now()-dateEnded)))*24 < 48
        and price>( select avg(price)* 0.50
                    from(select *, cume_dist() OVER (ORDER BY price desc)
                         from web_price_scan
                         where listing_Type='AARM'
                           and u_kbalikepartnumbers_id = 1000307
                           and (EXTRACT(Day FROM (Now()-dateEnded)))*24 < 48
                        )g
                   where cume_dist < 0.50
                 )
        and price<( select avg(price)*2
                    from( select *, cume_dist() OVER (ORDER BY price desc)
                          from web_price_scan
                          where listing_Type='AARM'
                            and u_kbalikepartnumbers_id = 1000307
                            and (EXTRACT(Day FROM (Now()-dateEnded)))*24 < 48
                        )d
                    where cume_dist < 0.50)
     )s

  having count(*) > 5

如果没有可用的值，如何让它返回0？

Answer 1

（添加此答案是为了为问题提供更简短、更通用的示例 - 不包括原始问题中的所有特定案例的详细信息）。

---

这里有两个不同的“问题”，第一个是表或子查询是否没有行，第二个是查询中是否有 NULL 值。

对于我测试过的所有版本，postgres 和 mysql 在平均时会忽略所有 NULL 值，如果没有什么要平均，它将返回 NULL。这通常是有道理的，因为 NULL 被认为是“未知的”。如果你想覆盖它，你可以使用 coalesce（如 Luc M 所建议的那样）。

$ create table foo (bar int);
CREATE TABLE

$ select avg(bar) from foo;
 avg 
-----

(1 row)

$ select coalesce(avg(bar), 0) from foo;
 coalesce 
----------
        0
(1 row)

$ insert into foo values (3);
INSERT 0 1
$ insert into foo values (9);
INSERT 0 1
$ insert into foo values (NULL);
INSERT 0 1
$ select coalesce(avg(bar), 0) from foo;
      coalesce      
--------------------
 6.0000000000000000
(1 row)

当然，“from foo”可以替换为“from (...这里的任何复杂逻辑...) as foo”

现在，表中的 NULL 行是否应该计为 0？然后在 avg 调用中必须使用 coalesce。

$ select coalesce(avg(coalesce(bar, 0)), 0) from foo;
      coalesce      
--------------------
 4.0000000000000000
(1 row)

Answer 2

我可以想到两种方法来实现这一点：

• IFNULL()：

  如果表达式为 NULL，IFNULL() 函数返回指定值。如果表达式为 NOT NULL，则该函数返回表达式。

语法：

IFNULL(expression, alt_value)

IFNULL() 示例与您的查询：

SELECT AVG( price )
FROM(
      SELECT *, cume_dist() OVER ( ORDER BY price DESC ) FROM web_price_scan
      WHERE listing_Type = 'AARM'
        AND u_kbalikepartnumbers_id = 1000307
        AND ( EXTRACT( DAY FROM ( NOW() - dateEnded ) ) ) * 24 < 48
        AND IFNULL( price, 0 ) > ( SELECT AVG( IFNULL( price, 0 ) )* 0.50
                                     FROM ( SELECT *, cume_dist() OVER ( ORDER BY price DESC )
                                           FROM web_price_scan
                                           WHERE listing_Type='AARM'
                                             AND u_kbalikepartnumbers_id = 1000307
                                             AND ( EXTRACT( DAY FROM ( NOW() - dateEnded ) ) ) * 24 < 48
                                         ) g
                                    WHERE cume_dist < 0.50
                                  )
        AND IFNULL( price, 0 ) < ( SELECT AVG( IFNULL( price, 0 ) ) *2
                                     FROM( SELECT *, cume_dist() OVER ( ORDER BY price desc )
                                           FROM web_price_scan
                                           WHERE listing_Type='AARM'
                                             AND u_kbalikepartnumbers_id = 1000307
                                             AND ( EXTRACT( DAY FROM ( NOW() - dateEnded ) ) ) * 24 < 48
                                         ) d
                                     WHERE cume_dist < 0.50)
     )s
HAVING COUNT(*) > 5

• COALESCE()

  COALESCE() 函数返回列表中的第一个非空值。

语法：

COALESCE(val1, val2, ...., val_n)

带有查询的 COALESCE() 示例：

SELECT AVG( price )
FROM(
      SELECT *, cume_dist() OVER ( ORDER BY price DESC ) FROM web_price_scan
      WHERE listing_Type = 'AARM'
        AND u_kbalikepartnumbers_id = 1000307
        AND ( EXTRACT( DAY FROM ( NOW() - dateEnded ) ) ) * 24 < 48
        AND COALESCE( price, 0 ) > ( SELECT AVG( COALESCE( price, 0 ) )* 0.50
                                     FROM ( SELECT *, cume_dist() OVER ( ORDER BY price DESC )
                                           FROM web_price_scan
                                           WHERE listing_Type='AARM'
                                             AND u_kbalikepartnumbers_id = 1000307
                                             AND ( EXTRACT( DAY FROM ( NOW() - dateEnded ) ) ) * 24 < 48
                                         ) g
                                    WHERE cume_dist < 0.50
                                  )
        AND COALESCE( price, 0 ) < ( SELECT AVG( COALESCE( price, 0 ) ) *2
                                     FROM( SELECT *, cume_dist() OVER ( ORDER BY price desc )
                                           FROM web_price_scan
                                           WHERE listing_Type='AARM'
                                             AND u_kbalikepartnumbers_id = 1000307
                                             AND ( EXTRACT( DAY FROM ( NOW() - dateEnded ) ) ) * 24 < 48
                                         ) d
                                     WHERE cume_dist < 0.50)
     )s
HAVING COUNT(*) > 5

Answer 3

使用coalesce

COALESCE(value [, ...])

The COALESCE function returns the first of its arguments that is not null.  
Null is returned only if all arguments are null. It is often
used to substitute a default value for null values when data is
retrieved for display.

编辑

下面是COALESCE 与您的查询的示例：

SELECT AVG( price )
FROM(
      SELECT *, cume_dist() OVER ( ORDER BY price DESC ) FROM web_price_scan
      WHERE listing_Type = 'AARM'
        AND u_kbalikepartnumbers_id = 1000307
        AND ( EXTRACT( DAY FROM ( NOW() - dateEnded ) ) ) * 24 < 48
        AND COALESCE( price, 0 ) > ( SELECT AVG( COALESCE( price, 0 ) )* 0.50
                                     FROM ( SELECT *, cume_dist() OVER ( ORDER BY price DESC )
                                           FROM web_price_scan
                                           WHERE listing_Type='AARM'
                                             AND u_kbalikepartnumbers_id = 1000307
                                             AND ( EXTRACT( DAY FROM ( NOW() - dateEnded ) ) ) * 24 < 48
                                         ) g
                                    WHERE cume_dist < 0.50
                                  )
        AND COALESCE( price, 0 ) < ( SELECT AVG( COALESCE( price, 0 ) ) *2
                                     FROM( SELECT *, cume_dist() OVER ( ORDER BY price desc )
                                           FROM web_price_scan
                                           WHERE listing_Type='AARM'
                                             AND u_kbalikepartnumbers_id = 1000307
                                             AND ( EXTRACT( DAY FROM ( NOW() - dateEnded ) ) ) * 24 < 48
                                         ) d
                                     WHERE cume_dist < 0.50)
     )s
HAVING COUNT(*) > 5

恕我直言，COALESCE 不应与AVG 一起使用，因为它会修改值。 NULL 表示未知，仅此而已。这不像在SUM 中使用它。在此示例中，如果我们将 AVG 替换为 SUM，则结果不会失真。将 0 加到总和上不会伤害任何人，但用 0 计算未知值的平均值，您不会得到真正的平均值。

在这种情况下，我会在WHERE 子句中添加price IS NOT NULL 以避免这些未知值。