如果返回值为null,则postgresql返回0

Question

我有一个返回avg(价格)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

如果没有可用值,如何使其返回0？

Answer 1

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.

编辑以下是您的查询的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

COALESCE不应该使用恕我直言,COALESCE因为它修改了价值.AVG意味着未知,没有别的.这不像是在使用它NULL.在此示例中,如果我们替换SUM为AVG,则结果不会失真.将0添加到总和不会伤害任何人,但计算未知值的平均值为0,则不会得到实际平均值.

在这种情况下,我会添加SUMin price IS NOT NULL子句以避免这些未知值.

Answer 2

(这个答案被添加以提供更短和更通用的问题示例 - 不包括原始问题中的所有案例特定细节).

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.

对于我测试的所有版本,postgres和mysql在平均时将忽略所有NULL值,如果没有任何平均值,它将返回NULL.这通常是有道理的,因为NULL被认为是"未知的".如果要覆盖它,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,"来自foo"可以被"from(...任何复杂的逻辑......)替换为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coalesce.

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