用COUNT（）计算

Question

我有一个可以分组的数据集，想知道每个组中行的百分比。

这看起来几乎可行，除了因为imho缺少类型转换而为每个组返回0

SELECT COUNT(*) / (SELECT COUNT(name) 
                    FROM x 
                    WHERE d = '0') 
  FROM x, y 
  WHERE x.a = y.a AND x.b = '0' 
  GROUP BY y.c 
  ORDER BY y.c ASC

如何正确投射？

Answer 1

您可以直接投射，而不是乘以1.0。这对我来说似乎更清晰，更清晰。首先，它清楚地表明了您想要使用的数据类型。您可能会对float4或float8近似值的精确度感到满意，而不是为精确的numeric计算支付额外费用。

SELECT COUNT(*)::float / (SELECT COUNT(name) 
                           FROM x 
                           WHERE d = '0')::float
  FROM x, y 
  WHERE x.a = y.a AND x.b = '0' 
  GROUP BY y.c 
  ORDER BY y.c ASC

test=# select 1.0 * 5 / 10;
        ?column?        
------------------------
 0.50000000000000000000
(1 row)

test=# select pg_typeof(1.0 * 5 / 10);
 pg_typeof 
-----------
 numeric
(1 row)

test=# select 5::float / 10::float;
 ?column? 
----------
      0.5
(1 row)

test=# select pg_typeof(5::float / 10::float);
    pg_typeof     
------------------
 double precision
(1 row)

Answer 2

你应该能够做到

select 1.0 * count(*) / .....

Answer 3

如果您转换为float，您的问题可能会得到解决：

SELECT convert(double, COUNT(*)) / (SELECT convert(double, COUNT(name)) 
                    FROM x 
                    WHERE d = '0') 
  FROM x, y 
  WHERE x.a = y.a AND x.b = '0' 
  GROUP BY y.c 
  ORDER BY y.c ASC