这是我的表'tab_test'：
year animal price
2000 kittens 79
2000 kittens 93
2000 kittens 100
2000 puppies 15
2000 puppies 32
2001 kittens 31
2001 kittens 17
2001 puppies 65
2001 puppies 48
2002 kittens 84
2002 kittens 86
2002 puppies 15
2002 puppies 95
2003 kittens 62
2003 kittens 24
2003 puppies 36
2003 puppies 41
2004 kittens 65
2004 kittens 85
2004 puppies 58
2004 puppies 95
2005 kittens 45
2005 kittens 25
2005 puppies 15
2005 puppies 35
2006 kittens 50
2006 kittens 80
2006 puppies 95
2006 puppies 49
2007 kittens 40
2007 kittens 19
2007 puppies 81
2007 puppies 38
2008 kittens 37
2008 kittens 51
2008 puppies 29
2008 puppies 72
2009 kittens 84
2009 kittens 26
2009 puppies 49
2009 puppies 34
2010 kittens 75
2010 kittens 96
2010 puppies 18
2010 puppies 26
2011 kittens 35
2011 kittens 21
2011 puppies 90
2011 puppies 18
2012 kittens 12
2012 kittens 23
2012 puppies 74
2012 puppies 79
这里有一些代码用于转换行和列，因此我获得了'小猫'和'小狗'的平均值：
SELECT
year,
AVG(CASE WHEN animal = 'kittens' THEN price END) AS "kittens",
AVG(CASE WHEN animal = 'puppies' THEN price END) AS "puppies"
FROM tab_test
GROUP BY year
ORDER BY year;
上面代码的输出是：
year kittens puppies
2000 90.6666666666667 23.5
2001 24.0 56.5
2002 85.0 55.0
2003 43.0 38.5
2004 75.0 76.5
2005 35.0 25.0
2006 65.0 72.0
2007 29.5 59.5
2008 44.0 50.5
2009 55.0 41.5
2010 85.5 22.0
2011 28.0 54.0
2012 17.5 76.5
我想要的是像第二个表一样的表，但它只包含第一个表中COUNT()至少为3的项目。换句话说，目标是将 this 作为输出：
year kittens
2000 90.6666666666667
第一张表中至少有3个'小猫'个体 这在PostgreSQL中是否可行？
答案 0 :(得分：12)
CASE 如果您的案例与演示一样简单，CASE语句将执行：
SELECT year
, sum(CASE WHEN animal = 'kittens' THEN price END) AS kittens
, sum(CASE WHEN animal = 'puppies' THEN price END) AS puppies
FROM (
SELECT year, animal, avg(price) AS price
FROM tab_test
GROUP BY year, animal
HAVING count(*) > 2
) t
GROUP BY year
ORDER BY year;
无论您是在外部查询中使用sum()，max()还是min()作为聚合函数，都无关紧要。在这种情况下，它们都会产生相同的值。
crosstab() 使用更多类别，crosstab()查询会更简单。对于更大的表，这也应该更快。
您需要安装附加模块tablefunc（每个数据库一次）。从Postgres 9.1开始就像这样简单：
CREATE EXTENSION tablefunc;
此相关答案的详细信息：
SELECT * FROM crosstab(
'SELECT year, animal, avg(price) AS price
FROM tab_test
GROUP BY animal, year
HAVING count(*) > 2
ORDER BY 1,2'
,$$VALUES ('kittens'::text), ('puppies')$$)
AS ct ("year" text, "kittens" numeric, "puppies" numeric);
此网站没有sqlfiddle，因为该网站不允许使用其他模块。
为了验证我的声明，我在我的小型测试数据库中运行了接近真实数据的快速基准测试。 PostgreSQL 9.1.6。使用EXPLAIN ANALYZE进行测试，最好是10：
使用10020行进行测试设置：
CREATE TABLE tab_test (year int, animal text, price numeric);
-- years with lots of rows
INSERT INTO tab_test
SELECT 2000 + ((g + random() * 300))::int/1000
, CASE WHEN (g + (random() * 1.5)::int) %2 = 0 THEN 'kittens' ELSE 'puppies' END
, (random() * 200)::numeric
FROM generate_series(1,10000) g;
-- .. and some years with only few rows to include cases with count < 3
INSERT INTO tab_test
SELECT 2010 + ((g + random() * 10))::int/2
, CASE WHEN (g + (random() * 1.5)::int) %2 = 0 THEN 'kittens' ELSE 'puppies' END
, (random() * 200)::numeric
FROM generate_series(1,20) g;
结果：
@bluefeet
总运行时间：95.401毫秒
@wildplasser（不同的结果，包括count <= 3行）
总运行时间：64.497毫秒
@Andreiy（+ ORDER BY）
＆安培; @ Erwin1 - CASE（两者表现相同）
总运行时间：39.105毫秒
@ Erwin2 - crosstab()
总运行时间：17.644毫秒
大比例（但不相关）的结果只有20行。只有@wildplasser的CTE有更多的开销和尖峰。
只有少数行，crosstab()很快就会占据主导地位。
@ Andreiy的查询与我的简化版本大致相同，外部SELECT（min()，max()，sum()中的聚合函数没有产生任何可衡量的差异（每组只有两行） ）。
一切都如预期，没有惊喜，采取我的设置并尝试@home。
答案 1 :(得分：4)
这是@bluefeet's suggestion的替代方法，它有点类似，但避免了连接（相反，上层分组应用于已经分组的结果集）：
SELECT
year,
MAX(CASE animal WHEN 'kittens' THEN avg_price END) AS "kittens",
MAX(CASE animal WHEN 'puppies' THEN avg_price END) AS "puppies"
FROM (
SELECT
animal,
year,
COUNT(*) AS cnt,
AVG(Price) AS avg_price
FROM tab_test
GROUP BY
animal,
year
) s
WHERE cnt >= 3
GROUP BY
year
;
答案 2 :(得分：3)
这就是你要找的东西：
SELECT t1.year,
AVG(CASE WHEN t1.animal = 'kittens' THEN t1.price END) AS "kittens",
AVG(CASE WHEN t1.animal = 'puppies' THEN t1.price END) AS "puppies"
FROM tab_test t1
inner join
(
select animal, count(*) YearCount, year
from tab_test
group by animal, year
) t2
on t1.animal = t2.animal
and t1.year = t2.year
where t2.YearCount >= 3
group by t1.year
答案 3 :(得分：2)
CREATE TABLE pussyriot(year INTEGER NOT NULL
, animal varchar
, price integer
);
INSERT INTO pussyriot(year , animal , price ) VALUES
(2000, 'kittens', 79)
, (2000, 'kittens', 93)
...
, (2007, 'puppies', 81)
, (2007, 'puppies', 38)
;
-- a self join is a poor man's pivot:
WITH cal AS ( -- generate calendar file
SELECT generate_series(MIN(pr.year) , MAX(pr.year)) AS year
FROM pussyriot pr
)
, fur AS (
SELECT distinct year, animal, AVG(price) AS price
FROM pussyriot
GROUP BY year, animal
-- UPDATE: added next line
HAVING COUNT(*) >= 3
)
SELECT cal.year
, pussy.price AS price_of_the_pussy
, puppy.price AS price_of_the_puppy
FROM cal
LEFT JOIN fur pussy ON pussy.year=cal.year AND pussy.animal='kittens'
LEFT JOIN fur puppy ON puppy.year=cal.year AND puppy.animal='puppies'
;