我有一个返回平均值（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？
答案 0 :(得分：145)
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
恕我直言COALESCE不应与AVG一起使用，因为它会修改该值。 NULL意味着未知，而不是别的。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，我会在price IS NOT NULL子句中添加WHERE以避免这些未知值。
答案 1 :(得分：15)
（此答案已添加，以便为问题提供更简短，更通用的示例 - 不包括原始问题中的所有案例特定详情。）
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，“来自foo”可以用“from（...这里有任何复杂的逻辑......）替换为foo”
现在，表中的NULL行是否应计为0？然后必须在平均调用中使用coalesce。
$ select coalesce(avg(coalesce(bar, 0)), 0) from foo;
coalesce
--------------------
4.0000000000000000
(1 row)
答案 2 :(得分：1)
我可以想到两种方法来实现这一目标：
IFNULL（）：
如果表达式为NULL，则IFNULL（）函数将返回指定值。如果表达式为非NULL，则此函数将返回表达式。
语法：
IFNULL(expression, alt_value)
查询中的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
COALESCE（）
COALESCE（）函数返回列表中的第一个非空值。
语法：
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