如果返回的值为null，则PostgreSQL返回0

我有一个查询返回avg（price）

  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

如果没有可用值，如何使它返回0？

使用合并

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

这是COALESCE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

IMHO COALESCE不应与之一起使用，AVG因为它会修改值。NULL意味着未知而已。这不像在中使用它SUM。在此示例中，如果将替换AVG为SUM，则结果不会失真。将总和加0不会对任何人造成伤害，但是对于未知值计算平均值为0时，您不会获得实际平均值。

在那种情况下，我将添加price IS NOT NULLin WHERE子句以避免这些未知值。

（添加此答案是为了为问题提供更简短，更通用的示例-在原始问题中不包括所有针对案例的详细信息）。

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在求平均值时都会忽略所有NULL值，如果没有要求平均值的值，它将返回NULL。这通常是有意义的，因为NULL被视为“未知”。如果要覆盖此设置，则可以使用合并（如Luc M的建议）。

$ 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（...任何复杂的逻辑...）as foo”

现在，表中的NULL行是否应计为0？然后必须在avg调用中使用合并。

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

我可以想到两种方法来实现此目的：

• IFNULL（）：

  如果表达式为NULL，则IFNULL（）函数将返回指定值;如果表达式为NOT NULL，则此函数将返回表达式。

句法：

IFNULL(expression, alt_value)

带有查询的IFNULL（）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 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

• 合并（）

  COALESCE（）函数返回列表中的第一个非空值。

句法：

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

查询中的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