仅使用最小 COUNT() 转置行和列（也称为枢轴）？

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

下面是一些转置行和列的代码，以便我得到“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

我想要的是一个像第二个表一样的表，但它只包含具有COUNT()第一个表中至少有 3 个。换句话说，目标是this作为输出：

year    kittens
2000    90.6666666666667

第一个表中至少有 3 个“kitten”实例。
这在 PostgreSQL 中可能吗？

CASE

如果您的案例如演示的那样简单，CASE声明会做：

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;

不管你是否使用sum(), max() or min()作为外部查询中的聚合函数。在这种情况下，它们都产生相同的值。

SQL小提琴

crosstab()

有了更多的类别，就会更简单crosstab()询问。这也应该是更大的桌子更快.

您需要安装附加模块表函数（每个数据库一次）。从 Postgres 9.1 开始，这很简单：

CREATE EXTENSION tablefunc;

此相关答案中的详细信息：

• PostgreSQL 交叉表查询